i have the same (or similar) issue. Do you run vista by any chance? As i do, and i believe that there is some issue with steam not recognising mics on vista
Have you tried right clicking on the little speaker in your taskbar (next to the clock in the bottom right corner) and choosing "Adjust sound characteristics" -> go into the 2 tabs called "Sound units" (or similiar) and "Voice" .. On all drop-down menus, choose your sound in-/output (the name of your headset, I.E).. There should be 4 drop-down menus where you can find your sound entity..
Click enter and restart CS:S .. Yes, you have to quit the game and enter it again..
USUALLY works for me.. CS:S will on launch, go search for the in- and outputs it "needs" and use those untill next launch of the game.. Of course remember to turn on voice input in the options in CS:S..
I am sorry for any slobby translations in the menus etc, but my system is danish and I just quickly translated it