To counter any display of ignorance in the previous post, I share this info from my brother, the self-described (and never modest) "internationally-known, well-respected medical professional" and, incidentally, father of four:

Doppler doesn't really sound any different on the inside. The sound waves it uses are undetectable by the human ear. So Cletus-the-fetus hears what you hear, only a little more muffled since there's a layer of Mom and amniotic fluid between. What Cletus is squirming from is the moron [in Cletus' world, anyway] poking Cletus in the side, the head, or whatever.