You don't need to put the plugs on your belly

for the peanut to hear the music. Just you singing or the music playing in the background is enough for LO to "hear" the music. Like ALM listening to music while exercising.

Along the same lines of "sounds", my DD can sleep through our barking Rottie but a creak in the floor boards can wake her and send her into a tailspin! I reason that it's because she was used to hearing the bark of our Rottie but she couldn't hear such "low" sounds like a a floor board creak.