Are you just super quiet. I used to be really really quiet in games with my mic because when you adjust your mic volume in audio options a USB mic will use a different input than Microphone.
So you have to adjust your volume for "WAV" sound, or "name of your microphone" rather than just the mini-jack microphone input button.
If all else fails you can remove and uninstall CS:S from your computer and re-install. If that doesn't work it probably isn't CS:S that is blocking your mic.
Sweet I'm glad you got that problem fixed... It was getting kind of scary what people were suggesting.. I was waiting for "if you throw your computer out the window it should work" lol..... any way my two cents...
BTW.. I was kidding... most of the suggestions were fine if not really good...