The following restriction exists for multicast music on hold (MOH) when a media termination point (MTP) is invoked.
When an MTP resource gets invoked in a call leg at a site that is using multicast MOH, the caller receives silence instead of music on hold. To avoid this scenario, configure unicast MOH or Tone on Hold instead of multicast MOH.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/4_1_3/ccmfeat/fsmoh.html
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_qanda_item09186a0080094766.shtml
Check list for configuring MoH multicast
Configure an audio source to allow multicasting.
Configure a music on hold server to enable multicast audio sources.
Create a media resource group and configure it to use multicast for MOH audio.
Create a media resource group list with a multicast media resource group as the primary media resource group.
Select the media resource group list created in Step 4 for either a device pool or for specific devices.