The command:-
musicPlayer.playFullFile("track001.WAV");
is what starts off the player. Therefore just take a code that turns on an LED when the button is pushed and replace the LED turn on bit with this line.
The command:-
musicPlayer.playFullFile("track001.WAV");
is what starts off the player. Therefore just take a code that turns on an LED when the button is pushed and replace the LED turn on bit with this line.