muting/volume adjustment for audio out

I'm trying to use Verilog to have my DE2 board play songs, and so far I've managed to figure out how to make different notes/tones, but the one thing I can't seem to figure out is simply how to make the board stop playing sounds.

Surely there must just be some kind of volume register or an audio reset/mute signal somewhere that I've missed? None of the 8 audio signals that are listed in the DE2 manual suggest that they control volume, as far as I can see, so I'm stumped.

Thanks everyone!