Also make sure the headset is set as your default recording device. Because ventrilo, in the options you can set the input device to your mic, as with CS:S,, it uses the one you have set to default.
How to set your mic to the Default Recording device:
Depending on your Operating System, things can be different. If you have Vista or 7, right click your volume control in the taskbar and go to "Recording Devices", Then select your microphone and push "Set Default".
If not on either of those, look around in your control panel for sound, then once in that, select the recording devices tab and do the same thing said above.
To test once done, use the test mic option in CS:S Options or do as matt said with "voice_loopback 1" in console and to disable "voice_Loopback 0"