They sing for their potential mates. You just can't hear it.
But this article also has sound files with a lowered pitch. They sound a bit like birds:

http://www.guardian.co.uk/science/story/0,3605,1605806,00.html

Sheesh. Tell us something we didn't know after watching Babe. That trio of singing mice doing all the intro's.

Elephants talk too yanno! You can't hear them either. Shame the same theory doesn't work on kids after a certain hour.

/me wanders off to the Random Complaints thread, cos son won't let her sleep.