Although it’s a completely different architecture, I experienced a similar issue (identical symptoms) on my Prophet-600 some years ago and replacing the 4051 did the trick indeed.
Swap the CEM chips about, and see if the fault moves with the chip.If it9;s the channel and not the chip, then replace the MC14051BCP (8 channel mux/demux chip) and/or the 74HC32N (quad 2 input "OR" gate) - worked for me on my M6 when a voice channel went mad.I have never personally seen a 'wide body' 3396 go bad, but the 'slim body' version as found in the M1000 has had a very poor record for me, with 5 out of 6 failing in my 1000 over time.