Hi Rob,
Here is some related info;
Multicast MoH from Branch Routers
Beginning with Cisco Unified Survivable Remote Site Telephony (SRST) Release 3.0, MoH can be multicast in a remote or branch site via the branch SRST router's flash. Beginning with SRST Release 3.2, MoH can be multicast in a branch site via a live feed connected to an analog port on the branch SRST router. Multicast MoH from a branch router via these two methods enhances the Cisco Unified Communications MoH feature in both of the following scenarios:
Non-Fallback Mode
When the WAN is up and the phones are controlled by Unified CM, this configuration can eliminate the need to forward MoH across the WAN to remote branch sites by providing locally sourced MoH.
Fallback Mode
When SRST is active and the branch devices have lost connectivity to the central-site Unified CM, the branch router can continue to provide multicast MoH.
When using the live feed option in either scenario, the SRST router provides redundancy by monitoring the live feed input, and it will revert to streaming MoH from a file in flash if the live feed is disconnected. You can use only a single multicast address and port number per SRST router to provide multicast MoH; therefore, the SRST router does not support streaming from both the live feed and the flash file at the same time. In addition, the SRST router can stream only a single audio file from flash.
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Note An SRST license is required regardless of whether the SRST functionality will actually be used. The license is required because the configuration for streaming MoH from branch router flash is done under the SRST configuration mode and, even if SRST functionality will not be used, at least one max-ephones and one max-dn must be configured.
From this doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a0080637422.html
Integrating Cisco CallManager and Cisco SRST to Use Cisco SRST As a Multicast MOH Resource
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00802d1c31.html
Hope this helps!
Rob
