This is the method for playing sounds (provided you have got access to the pointers):
if (sfxPlayer != null) {
sfxPlayer.play(sfxLibrary.get(sfxLibrary.ILLEGAL_MOVE));
}
if (musicPlayer != null) {
musicPlayer.play(musicLibrary.get(musicLibrary.INTRO));
}
Notice the if-statement checking wether or not the sfxPlayer-object is null. Beeing null indicates that sound is disabled.