This PlayerHater library is also one that facilitates the handling of playing music in your app. Moreover, it gives you the mechanism to also let the music be played in the background. Check it out here: https://github.com/PRX/PlayerHater
This whole set for Android provides you an audio infrastructure representing a simple, callback-driven API for implementing audio modules (such as synthesizers and effects), a graph-based API for connecting audio modules, plus support for inter-app audio routing. A very cool tool for creating your own music-based apps. Check it out here: https://github.com/google/patchfield
This AudioWife library is a very handy library representing an audio player. This library will help you save many hours of development time to figure out how to develop your own audio player. Check it out here: https://github.com/jaydeepw/audio-wife
This AACDecoder Android library is probably one of the best for those desperately looking for handling raw AAC files. This library will also save you hundreds of hours of development. Check it out here: https://code.google.com/p/aacdecoder-android/
This very well-crafted Rebound library developed by Facebook offers you the possibility to easily create animations that simulate natural world physics to your app. This is one of the very few libraries saving you hundreds of hours of development time. Check it out here: https://github.com/facebook/rebound