ill bet this is a mixer deal, not a cep deal.
how is your mic routed? Is it going into ch 1 and then the Main outs are going into your computer? Ill bet it is. If you have an AUX SEND or something, you need to route it through that, and route your Computer's signal through the mains into your headphones. Or vice versa. whatever you do, you need to NOT route your computers signal through ALL the same things your mic is going through.
does that make sense?
Then again, your soundcard could be a cheap peice of crap thats only half duplex. I bet thats it...
xoxo