Setup in AUM- AU Viking synth on input, IAA output port 1
Setup in MTD- input AUM IAA port 1, input monitoring on
When recording in MTD, I switch to AUM to play Viking with AUM keyboard. I can hear the MTD metronome as I record,but not the synth.

I can do this setup using Audiobus instead of AUM and can hear Viking as I play and record.

Am I doing something wrong?

It seems to be working. The only thing is maybe you have Punch and Auto Input turned on in multitrack? That would cause input monitoring to turn off when the playhead is outside the punch markers.

Yes! That was it…thank you!

No problem, thanks for letting me know