Sound waves can tweak physiology. Sound has been used for healing but also for warfare, crowd control and the like. Especially considering that the baby is trapped in there and can’t leave the sound enviroment if it is causing the baby any discomfort, I don’t think that is a good idea. Listening normally (i.e. to speakers in the room) is probably fine, as long as the music isn’t too harsh or loud; but right on the stomach like that, I don’t know.