It seems that you've accidentally muted the game's audio. Just press "M" to undo it. (It happens all the time.)
This is incredibly useful. Thanks! 
(i thought it was a bug xD)
It is a bug, and was supposedly fixed in 2.2... except it wasn't.
EDIT: oh, you wasn't even using 2.2 in first place -- but regardless, the bug was never fixed.