Just modify your MXF clips before you create your multi-cam sequence. You can "lick" them before creating the sequence :-)
If you don't actually need all the channels of audio from each camera, just right click on them in the project bin and choose Modify / Audio Channels. Change from 5 Audio tracks to 2 and choose the format (mono / Stereo) you want. Once you have modified all cameras, then select them all and create your multi cam sequence.