You're running into an issue where the SRST Multicast MoH feature to save WAN bandwidth only lets you set one IP multicast group and not multiple and has no way to detect a CCM failure really since the 2 don't work together and are seperate configurations. It's always going to stream to the 1 multicast group.
This was more of a neat trick to save WAN bandwidth really. Since it is multicast phones just are tuning in to a mcast stream and that mcast group address is controlled by call manager. But the source doesn't need to be call manager server and can be anywhere on network just as long as the phone can tune in when it is put on hold. In your case, it is the router sourcing the stream.
Ideally for sites where you are doing this, you want to set the MRGL for those users/devices to always use same MOH server/audio source. This way the address is always the same in the phone config since router can only have 1 mcast stream configured. I'm not sure what happens if the MoH server fails on CCM server if it will instruct phone to use that mcast group address or something else during failure of services. Haven't tested that scenario with this yet. You could do unicast on other server for backup then phone would get instructed to use unicast stream during failures but that would use WAN bandwidth then at those times.
