From what I gather, this may be "normal" that the music slows down. How to do synchronize the music with the frame flyback? In other terms: in your menu, how MUCH do you wait before calling the next frame of the music player? And HOW do you wait?
During your game, if moving your sprites takes too long, and you play a frame of music afterwards, it will feel slower, of course.
What you must do if put the music under interruption. Maybe AGD has a function for that? Basically, you must play the music every 50hz. If you don't, the music will be faster or slower. Using interruptions will allow you to play at precise occurrence, instead of "when the CPU has time" or "when my game loop is over".