I did not put music diectly on by belly for a good reason. How do you know you are not disrupting their sleep cycle? (unless you are FOR SURE they are awake) but, how many times are you going to say "OK! they are awake now, let me go find my belly phones!" Many parents put the head phones on when it is convenient for themselves, not their babies. This is very bad (in my unprofessional opinion) because it could wake them from a very important sleep cycle.
Why on Earth would anyone want to bombard their child with the noises they can hear the rest of their life and rob them of the peaceful noises they can only hear for 9 months? Infants are the most soothed by the natural sound of mama's belly, which is why many people record thier belly sounds during pregnancy. I would recommend this instead.
anyway, just my opinion. God bless, liz