I believe that by changing the sampling rate, you can change the play speed. Double the sample rate to double the speed, or decrease it to its half to obtain half the speed, and so on. If the mp3 library you're using doesn't allow you to set the sample rate, switch to another library, or convert the mp3 file to a wav file and use AudioStreamer from Audio library to play it.
You can change the sample rate with. Audiostreamer or Audiotrack, but be aware that the pitch of the playback will also change. If you want to change the speed without affecting the pitch you would need a specialist library like surina's Soundtouch, which.is.not yet available on B4a.