If the pc card headphone amp is amplified, watch out for levels! Some soundcards like the old blasters had specific headphone outputs that passed thru their own headphone amplifier. This was meant to power small headphones and unpowered computer speakers. Since headphone amps require line level signals, you could easily overdrive their inputs by using the card's headphone output. I suggest using line level outputs or the control room outs on your mixer. You can even use the mixer's aux outs if monitoring in mono when using headphones isn't an issue.
