Hello,
With the current info, this can be anything from ESD, transient phenomena during power up/power down, use outside typical operating conditions, etc.
Just some things:
-If the mixer functions, are voltages, currents, etc as expected?
-Does the mixer fail during operation (with the main board), or immediately after cycling power or other repeatable issue?
-Do they also fail when running in the lab (under controlled environment, free from ESD isssues)?
-Do coupling capacitors towards the ports have correct value (and is this value really present on the circuit boards)?
-Are all supplies stable (so do they supply correct voltage/current and aren't oscillating)?