bug apparently with Siri which has several bugs regarding volume. play a second of iTunes music and the sound will reset to normal. happens to me half a dozen times per day. been looking for a cydia hack to actually FIX this until apple finally does. (don't expect a quick fix; version 5.1 of firmware STILL doesn't have a SCALE on the MAP and mapping to a lattitude longitude doesn't work because ignorantly map app adds 'united states' to ALL addresses unless specified (so stooopid).
double-tap home, swipe right will get you quickly to play/pause button to fix the sound.