One of the unsung apps that exists out there and it got updated recently again with ​Multitouch support and Sidechain compression. Did not mention the last update that also made it possible to control it with midi. Unfortunately not to play notes with it but it is great for direct manipulation and it is possible to control it with for example G/Stomper, This I have not tried yet and do not know how that works tough. 
In either case this is one cool and multi faceted sample instrument with a shitload of effects and possibilities. Plus being able to record/load into four different channels (but pushing it fully a powerful device is needed).
Have written about this some times now but if you never saw thesse posts think it is always good to bring it up again...

Google Play:
​play.google.com/store/apps/details?id=me.rocks.grainstorm

Use as/for:
- Sound design
- Experimental musical instrument
- Fun
- An alternative to pre-recorded music.

Key Features: 
- Four tracks
- Full control over the source sound
- Infinite options to shape the grain. 
- LFOs and Envelope Followers
- Handmade effects
- Realtime Crossynthesis 
- Time Stretching, Phase Vocoding
- Supports sound files of up to seven minutes length in any audio format 

First use instructions: 
1. Press POWER. Upper right power button turns on audio engine which is TURNED OFF on startup to save battery.
2. Press EJECT. Load a sound file. 
3. Press PLAY. 

Further instructions in the menu.

Recommended device specifications:
- 64bit CPU
- 2GB+ RAM
- Android 8.1+
Not recommended for low end devices.

This is about to get released during 2019 but the developer want to Beta test some more.

It has been released on iOS for awhile and got a lot of praise.

​Have been testing it and like it a lot.

It is easy to use but can get complex, intriguing and useful results.

It is midi compatible which is great as it is an app that can give unique timbres that is hard to get with traditional synthesis. Had to use a Midi to USB and then connect it to a USB OTG cable on the device used.
​(If not having an Midi to USB cable and thinking of getting one, please investigate a little bit before buying, there is cheap ones that do not work... Chinese, Ebay, Amazon etc...). 

The app is a lot of fun at the expense of CPU... So a good device is needed with Android Oreo or later.

Think to get an idea of the app take a look at the videos down below...

To get into the Beta write the developer here:
deltavaudio@gmail.com

​Website:
www.deltavaudio.com/

It will be one of the big releases 2019 for sure.

Feel that this app do not get the attention it deserves.
It is strange that there is not more than fifty people that paid for the full version!

It has grown significantly since it first came. Also in the beginning it was highly unstable now it is stable and have a lot of functions that makes for the possiblitiy of very deep sound manipulations.
There are somethings that are not so obvious if not taking some time with it,
but it is pretty easy and fun to use after just an hour or so.
So recommend this for anyone that are into sound manipulation and experiments.
Happy to see it being updated regularly now (specially as hardly nobody paid for it since it came out over three years ago) and am curious in what will be added to it in the future as it already have some unique features.

Google Play info:

Grainstorm is a Granular Synthesizer/ Phase Vocoder with Crossynthesis functionality.

It can be used as/for:
- Sound design
- A standalone experimental musical instrument
- Fun.

Features: 
- Ultralightweight design
- Four audio tracks
- Crossynthesis: Filtering output of one track by output of another track on a per grain basis.
- Cascading multiple tracks by the above mentioned effect.
- Spectral phase vocoder effects. (Only a few, simple ones right now. Probably more in the future.)
- Source files of up to seven minutes length in many different audio formats supported.
- Audiosource can be zoomed in and out until one audioframe per pixel is displayed to determine the exact position from where the grain is to be synthesized.

First use instructions: 
1. Connect device to fat amp.
2. Choose input either directly recorded by pressing microphonebutton or prerecorded audiofile by pressing ejectbutton. (Audiofile loading requires unlocker.)
3. Move marker on the right side of waveform to determine playbackposition.
4. Press playbutton. Should playback sound in original form.
5. Enter settings by pressing the left sidepanel. To modify playbackspeed etc.

Please report bugs, opinions and suggestions to grainstorm@rocks.me

Note: On some devices audio clipping may occur or controls may not work.

For free (and no publicity) granular goodness this is a hands on and simple app.

Google Play:
https://play.google.com/store/apps/details?id=com.FormerSpaceMarineSoftware.Granular&hl=en

The interesting developer/s Former Space Marine Software have just released a new granula app that comes with just three controls but it is possible to coax out a lot of variety and timbres out of it. It comes with some samples that are pretty useful and there is possibility to move samples into a SD-card or into device memory. Which is for me important when it comes to granular apps so it is possible to get the right tuning for using with compositions.
Recommended for anyone looking for some glitchy/experimental sound manipulations to play with. And yes for free with no publicity!

Google Play info:

Start making the grainy soundscapes of your dreams with this sound-mangling granular synthesizer. Granular synthesis takes grains from existing sounds and spits them out in new combinations to make new sounds. Use your finger to play your own audio files or just use the ones that come with the app.

https://play.google.com/store/apps/details?id=com.FormerSpaceMarineSoftware.Granular

After laying dormant for years after first release it got updated and became much more useful. Now already after a short time a second great update as it now can load four sound sources on different tracks!
Remember that it can also do direct recording. So if you have any interest in granular synthesis or experimental sound design this is kind of a must now as there is not anything close to this on Android. Plus being a lot of fun! If it is confusing at first do not give up as it will soon yield its secrets with some persistence.

Google Play:
​https://play.google.com/store/apps/details?id=me.rocks.grainstorm

For a long time wanting a granular application to play with on Android and one came out about two years ago. This was good and all except that the direct recording did not work and it could only support Flac and MP3 files. Which led to me not using it all as having to constantly having to convert files kind of is too much for a lazy person like me. The developer did not respond either so kind of gave up that something would happen after a year...

​In either case good news!
Now it imports wav files and the most fun of all is that the recording works now!
So if there is experience with granular apps from other platforms all the main controls are in this app but if not having used granular apps before it is a slight confusion at first. 
Not too crazy complicated though.
The fun thing for me is to do direct recording and record from the device into existing recording/compositions as there is a lot of fun things to be had that way.
Would recommend this to anyone looking for textures of different kinds or just sounds that can not be gotten in other ways.

Happiness is new sounds!

Google Play:
https://play.google.com/store/apps/details?id=me.rocks.grainstorm

So for a long time was waiting for a granular sample manipulator and found this one like two months ago...but...
Would have liked to have done an review but as the direct recording did not work and the developer do not want to answer my emails feel that I can not do an review as I do not know if the developer is looking into the issue of failed recording or what is the future plans for the application... To his defense he does say to test with demo version first in the Playstore description so if you want to try it that is the way to go.
Yes you can load samples but strangely enough only Flac or Mp3's in stereo formats. So to use this and if you are like me and the direct recording does not work (Samsung Galaxy S3 Android 4.3) you are going to have a media converter  installed to convert your audio files into Flac or Mp3.
Well outside of my big disappointment of not being able to record directly it does a nice job of granulating your sound files. 
As this is not going to be a review will just post the Playstore information and if you like me have been looking for some granular fun on your device and not having any other option you might want to look into this one? Except the direct recording and to only be able to manipulate Flac and Mp3 files it does work very good.
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Realtime Granular Synthesis for Android.
Grainstorm takes sound data as input, granulates it and pushes it back to audio out in realtime. 
Input either comes from recordings over the internal mic or from audio files which the user can choose from disk and are decoded afterwards. Currently only .mp3 and .flac stereo formats are supported, the samplerate of the input has to match the samplerate of the device (which is determined automatically and is 44.1kHz in most cases) or the result will be time-stretched or time-shrinked respectively and the input file must not exceed a length of 7 minutes. 
There is a cursor which can be moved over the waveform to determine the exact position from where input is taken. The waveform can be zoomed in until one sampleframe per pixel is displayed. Thus the cursor can be moved as exactly as one sample if required.
The two other cursors determine start and stop position of playback. In combination with playbackspeed and the other parameters evolving sound textures, morphing over a short or long period like hours can be created.
Interesting results - always depending on what you take as input - which could be used for electronic music production can be achieved if input comes from sampled voices or synthesized sounds samples.Output can be recorded by pressing the record button. On the second press the recording is automatically saved in .wav format in the user's music folder.

Grainstorm performs pretty well on recent devices with multi-core CPUs. The DSP-part which is highly optimized and most of the GUI-part are written in C - interaction between both is fast.

Before downloading the unlocker you should make sure that Grainstorm works satisfyingly on your device. It is not recommended for "slow" devices. If you encounter clipping turn out all background services. If you still encounter clipping, then Grainstorm might not be suitable for your device.
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Playstore link:
Grainstorm