Is your friend's voice so high-pitched that people are starting to stare? Sweetly ask, "Can you modulate your voice, please? To modulate is to change the pitch of something.
You can modulate things other than sound — it still refers to something that's being adjusted. Schools might modulate the number of students in the hallways at the same time by having each grade level start and end each at a different time. Traffic lights can modulate the number of cars that pass through an intersection in a certain interval. Computer modems modulate signals to allow computers to transfer information. In fact, modem gets its name from modulate/demodulate.