When you do overhead paging with background music, and supply music on hold, Asterisk can do some weird things.

Someone placed on hold told us afterwards that the hold music was 'too loud' and sounded like crap.

Turned out, the MOH was mixing with our background music coming out of the speakers, and mixing with our paging! I had to stop using mpg123 for Asterisk and manually convert my hold music files over to .wav format to force Asterisk to process them internally. This article at AstRecipies describes how to do it.