For smaller audio files, take a look at the Opus codec. It does way better than mp3 at lower bitrates. Android 5.0 and up can decode it natively and there's a library with NDK that should work on older devices.
Awesome, I'll take a look. My main reason for choosing mp3 was compatibility, but I didn't research it thoroughly, I just chose the cookie cutter option. My app is currently min 5.0 so this seems like a very viable option. Thanks :)