It's impossible to tell you from here what to turn up. The answer is "turn up the one that's not feeding enough signal to the next piece in the chain". Just to make sure, your sound source flows as follows:
mpc -> mixer -> sound card
You need to make sure that the output from both the mpc and the mixer is "healthy" but not distorting. If the MPC is feeding the mixer too low of a signal, turn that up. Same thing for the mixer feeding the sound card.
If both of those are sending a good signal, there's probably some input level setting you can adjust on your sound card (I don't know a thing about the audiophile card).
Hope this helps you track it down.