I had the same thing like a week or two ago.
Go
here.
Download the Quake Sound Pack or, as they have it listed, "Quake Pack Sounds" -- third link down.
Extract into the appropriate directories -- don't have Steam on this box or I'd tell you exactly -- but if I remember correctly, the directories in the zip match up to some folder inside Program Files\Steam\ -- I think it was like games\CounterStrikeSource or whatever
If you extract it right the files will now be in the /sounds folder.
Restart steam, and you'll be good to go.
Post if this doesn't work or you can't figure out my weak directions