Try to do Acoustic Echo Cancellation on Android with AEC modules from Speex and WebRTC.

Illustrating Doppler effect using sounds.

Emitting sounds of user-specified frequencies

breath Respiratory rate monitor based on smartphone using acousitc signal

display spectrum of ambient sounds

Acoustic Field simulation for phased-array controllers https://www.youtube.com/watch?v=vAEZvYlUnEM

Simple networking using microphone and speakers