I do have a Soundblaster 5/rx with a line-in, my mixer has an aux-return (and if that does not what I think it does, it has a headphone out too which I could use).
The signal that is sent to the speakers and the signal that my mic (connected via USB) picks up is different of course. But the difference should be the same all the time, my microphone does not move.
What I'm currently trying to figure out is if it is possible to subtract the signal that goes to the speakers (music, game noise & other voices from the mixer) from the signal that is handed to communication software. So my mic picks up everything including my voice, some black box removes what my speakers receive (factoring in distortion by playing through speakers and re-recording) and the result (just my voice) will be left over. Is this feasible or just some pipedream?