A USB headset is a separate audio device, but not a sound card. Stereo mix in your built-in audio sound card can only record gameplay audio playing on that built-in device. A USB headset won’t usually have an option to record its own playback.
It’s unlikely you can record 7.1 audio in Audacity on Windows. Stereo Mix is what it says - stereo (2 channels). I am not a gamer, you would need to ask game specific questions on a gaming forum.
Yes you can record from the USB headset mic on its own if you set the headset as the recording device in Audacity Device Toolbar .
If you want to record gameplay audio and your voice at the same time, then assuming you are on Windows Vista or later a solution would be to use Fraps to record the game audio and your microphone. This should work with a USB headset. See: http://www.youtube.com/watch?v=VMTGy-wbdSU .
You will probably need to turn down the volume of the in-game sound so that your microphone can be heard.
Another alternative would be to use a wired headset with pink and green connectors that uses your built-in sound device. Then if you unmute the microphone in the Playback section of your sound control panel (or on Vista and later, open Windows “Sound”, go to the Recording properties of your wired headset and choose “Listen to this device”) you can record the game and your microphone in Audacity using stereo mix.
“Listen” will have some delay so the mic may be recorded slightly late.