Think that this has been going on for over a month already. Noticed that they stopped doing the daily free app and guess that this is some kind of version of that.
In either case the apps that are for free are full paid apps and there is some good music apps to be had! Plus that they are I guess there permanent or at least for a longer time.
The procedure is to download a Amazon Underground APK and install after this it is possible to download the apps as normal.
What stopped me though is that there is a lot of permissions that for privacy reasons seems a little bit too much but I guess that they are not worse than Facebook?
So I did not do it and also had the apps that I was interested in...
The apps that I like that are for free are:
G-Stomper VA-Beast Synthesizer
There is also some vocal training apps that looks interesting but that I have not tried plus an assortment of other things... to see the music apps that are there:
Amazon Underground Music apps
To download the apk as to be able to download apps or see the other free apps: