Interesting side effect of using Lua for Android app is that anyone can modify source code on the phone and see the changes on next launch. This allows users to add instruments, modify existing ones, or change any other aspect of app.
The app is light on GUI and options to make it accessible to very young children. Please show it to your kid and report results :)
BTW does anyone have advice on how I can reach interested audience for this non-commercial project without spending money or too much time? Despite good users reviews I didn't see much organic growth in past months.