You can specify alternate MOH sources on the System | Telephony | Tones and Music page.
You can assigned the alternate sources as the Hold Music Source for an Incoming Call Route or a Group.
That assigned MOH source overrides any current MOH source associated with the call.
The assigned MOH source remains associated with the call as it moves around the IP Office system. This is done using the number of the MOH source (with 1 being the number of the default system source).
If the call moves to another IP Office system in a multi-site network, the source with the same number of the other system is used if also configured on that system.