solution for no sound from USB DAC (may or may not be specific to the MOTU M2)…
downgrade alsa-ucm-conf to 1.2.7.1-1 - this may be enough by itself to restore sound, but if not…
downgrade alsa-lib to 1.2.7.1-1 version
in my case power cycling the DAC was enough to restore sound (no reboot)
references…