GML Script. By using this after our setup_listeners(); gist, you can play audio on any listener, fade over time, and loop.

audio_play

///audio_play(listener,snd_filename,fade in time[ms],loop?)
var listener = argument0;
var sfx = argument1;
var time = argument2;
var loop = argument3;
if gamepad_is_connected(listener)
{
    sfx = audio_play_sound_on(listener, sfx, loop, 0);
}
else if listener == noone || listener == -1
{
    sfx = audio_play_sound(sfx,0,loop);
}
audio_sound_gain(sfx,global.sfxvolume/100,0);
audio_sound_gain(sfx,global.sfxvolume/100,time);