In short, Moh isn't playing when a subscriber is the MoH server...
We copied a custom Moh file to the publisher via "upload file" in MoH audio source. Two of our subscribers have "Cisco IP Voice Media Streaming App" and we've added them to the appropriate MRG/MRGL. Tests fail. If we enable Cisco IP Voice Media Streaming App on the publisher and add it to the MRG, reset the Moh server it works. Note that the defaul, built-in Moh files play just fine from the subs.
Any assistance appreciated...
You need to upload the MoH file to all CUCM servers acting as MoH servers. The files are not replicated from the Publisher.
This caught me out too in my first CUCM 5 install.