First, I am no lawyer, so take this as just a guess:
That depends, I think: If you use Android's built-in tools, I think you are free and clear (
http://developer.android.com/guide/appendix/media-formats.html). If you use a library that works on mp3 format, check to see if that library has and can transfer license to you via use of its library. If you roll your own, then you probably need to consult someone in legal, because I think the licensing for mp3 is pretty complicated (who owns patents, where they are enforceable, when they expire(d))...