Call onComplete if _sound is null

If _sound is null then return is simply called

function startSound(st:Float):Void {
	if (_sound == null) return;

	_time = st;
	_paused = false;
	_channel = _sound.play(_time, 0, _transform);

	if(_channel != null) {
		pitch = _pitch;
		_channel.addEventListener(Event.SOUND_COMPLETE, stopped);
		active = true;
	} else exists = active = false;
}

Because of this some necessary functions aren't called
I suggest just calling these functions and I think it will be enough

if (_sound == null) {
	if (onComplete != null) onComplete();
	cleanup(autoDestroy);
	return;
}

Start by locating the startSound(st:Float) function and trace how onComplete and cleanup(autoDestroy) are normally invoked. Verify the null-_sound path invokes both before returning, and check the related sound behavior to confirm completion and cleanup occur as expected.