babies do respond to sound from outside the womb. There are studies showing they open their mouths more when she sings a lullaby!
But another study says “when fetuses were exposed to a tape recording of their mother reading a passage, they showed a 5-bpm increase in heart rate over the first 20 s following voice onset, an increase that was sustained until the end of the recording span. In contrast, they observed a decrease in heart rate of 4-bpm when fetuses were exposed to an unfamiliar women’s voice reading the same story passage. “