This is a useful video. He does speak about being unable to sync the Model 12/MPC, but I think he's missing that the Model 12 will act as a sync master.
Unfortunately though, the MPCs will not record audio when in slave mode. My imagined but as-yet untested workaround, when it comes to recording audio stems from the MPC to the Model 12 would be: (1) set timing internally on the MPC (ie not in slave mode) and do your recordings (loops, etc etc) and arrange your sequences/song. (Audio can still be routed in/out via the Model 12 if desired)
(2) Consider leaving a bar or two empty at the very start, to allow later MIDI sync time to lock in.
(3) Set Model 12 BPM to the same as the MPC and have the Model 12 send sync (clock or MTC)
(4) Set Model 12 to record, and trigger playback on the MPC (given that you will be triggering 'record' on the Model 12, you might have to hit overdub/record on the MPC to take it out of record mode and leave it in playback. And/or silence the metronome, or route it somewhere it won't be recorded!)
(5) You should now have a sync'd recording on the Model 12, give or take a few ms latency (and hopefully without too much 'timing drift' - as I say, this is untested as yet!)
The advantage of having this is that any subsequent recordings can be sync'd to the Model 12 if needs be, whether or not the MPC is involved.
