When I still played shooters online:
Online Source games always had me mute everybody who talked. There's a simple reason why an integrated voice service is horrible.
A more personal reason would be that I like to hear the game sounds. Not the gunshots(well...) but the sound of someone landing on the floor behind me while I'm sniping, the sound of someone running six rooms from where I am, the sound of and delay between bullets anywhere on the map and those are only the obvious sounds.(for comparison, Windows sound volume is set to 10%, my headset is at roughly 40%, winamp is at 50% and it's still too loud in some cases so yes, I can hear what someone does on the other side of the map as long as the game doesn't have a limit)