US-122 with analog mixer

[pcp]

im using the us-122 basically just to recieve and to send back audio to and from my computer...i got an analog mixer plugged into the Line/Guitar In L and R inputs and the RCA line outs on the 122 going to an input on the mixer and the output on the mixer going to an amp. i make beats using reason for my recordings but what i want to do is just send the vocals and the guitar or whatever the case may be that i record (using the mixer) back to the computer...i dont want to send the beat that is coming through the 122 into the mixer back as well. basically how i do it is i use cubase sx and i put the beat in track 1 and record the vocals using the 122 inputs to tracks 2 and 3. im not sure if that made any sense to anybody else, but is there a way to mute the beat coming back through in the recording but not in the headphones so that i can hear the beat while i lay my vocals down?

[pcp]

does anybody out there have a mixer hooked up to the us-122? if so, how do you have it configured?