When monitor is off cubase fails to pick up the synth
Well that’s exactly what is supposed to happen…with monitor on the input is monitored (live played synth). With monitor/record enable off you should be able to hear the recorded synth track.
The thing is that if you can hear the MPD track playing (or more accurately the instrument it has programmed) there is absolutely no reason you shouldn’t hear the Audio track too. Record enable and Monitor button need to be off…everything else as already stated.