Can you play a mp3 file from the assets folder?

You can do it in another way too. Put the .mp3 files under res/Raw folder and use the following code:

MediaPlayer mediaPlayer = MediaPlayer.create(getApplicationContext(), R.raw.android);
mediaPlayer.start();

private void startSound(String filename){
    AssetFileDescriptor afd = getAssets().openFd(filename);
    MediaPlayer player = new MediaPlayer();
    player.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());
    player.prepare();
    player.start();
}

You can use the following code to play a specific audio file (i.e your_file.mp3) in your assets folder with:

MediaPlayer mediaPlayer = new MediaPlayer();
AssetFileDescriptor afd = context.getAssets().openFd("your_file.mp3");
mediaPlayer.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());
afd.close();
mediaPlayer.prepare();
mediaPlayer.start();

And to play a raw resource file (i.e your_file) with:

MediaPlayer mediaPlayer = new MediaPlayer();
AssetFileDescriptor afd = context.getResources().openRawResourceFd(R.raw.your_file));                    
mediaPlayer.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());
afd.close();
mediaPlayer.prepare();
mediaPlayer.start();