I've always wondered that! I'm guessing what they hear like are voice and maybe the tiny bit of light they see lol
Thats a good question lol
Lol. Yea maybe light to and music but they dont know what music or light is.
They say babies remember and respond to music they hear in utero plus our voices they can recognise while in the womb so I would say they dream of what they hear :) but of course no one will probably ever know for sure