Soundcard is fine.
Try to get your microphone at the same distance from your mouth all the time, as well as not to close and below it. Microphone below mouth at the same distance all the time, then regulate the loudness of the microphone so it doesnt overspeak... try ask a friendly ah player to judge.
Most times when someone is to low in volume its because the microphone slipped away, when he sounds verry squeeky its microphone or turned to loud, when its bad it sometimes sits to high.
Also note that you speak diffrent into the microphone than you normally do, if you have someone in your room ask him to listen halve an hour while you play. Some people put up a coarse voice, others talk to loud or mumble when on microphone.
Me, for example i sometimes tend to tighten up my throat up to a degree that its totally impossible to understand me, when i notice that i tension up more and no one can understand... then i need to take 5 mins of vox break and it is like normal again. I can assure you that i have no problems talking normally, but somehow on the game it gets me
.
Also might try to put some foam over the microphone, makes it better sometimes.