Can't hear any sounds in multiplayer

I can’t hear any sounds when I play on multiplayer (i.e. ONLINE on SERVERS), BUT, I can hear all sounds just fine on single player. Anyone know any reason why this isn’t working?

Note: After awhile, in single player, all of the sounds will stop as well.

System Specs:
OS: Win7 Home Prem. 64-bit
Vid: ATI Radeon 5770 (no crossfire setup)
Proc: i7 2.6Ghz
RAM: 8GB DDR3

DELL Studio XPS 8100

Addons:
WireSVN 2709
hoverboard
SimplePropPRotection
Slammer
StackerStool

EDIT/WORKAROUND: If I go into multiplayer and change the sound quality, I can hear again. I just have to change it from High to Medium…hit apply…and and change it back to High, and hit apply. Now, it doesn’t have to be high, but I prefer it.