I've been looking around and so far had no luck finding an 8.1 bug for MOH.
I've got a customer with a UC560 that is unable to get the external MOH jack working. I tested it myself from my blackberry so I know the music source is good. I've changed the setting under features in CCA to external music source and tested the hold music with both internal and external calling and all I get is this awful tone.
On a side note: I've scheduled an upgrade to 8.2 next week anyways because they need the extra licensing for additional sets.