it does not appear as though that device allows multichannel I/O. simply just stereo via the rec out.
this from yamaha site...
"In addition to the normal stereo buss, the MW12C and MW12CX has assignable busses (REC OUT). This feature gives you addes flexibility in routing the signals for recording and monitoring. Turning on the ON switch for any channel immediately routes that channel to the USB terminal and enables it for recording. Multiple channels selected in this way are, of course, recorded to a stereo mix on the computer. In this condition, each channel is also mixed to the stereo buss and can be monitored (with the Monitor Mix function)."