Multicast Audio 6 seconds delay

Have been trying to setup Cisco CallManager Music on Hold (MOH) to do multicasting on the network.  Had issue with the music takes about 6 seconds to be heard.  It doesn't matter, whether used spare or dense mode on the network.  It turns out a Cisco bug CSCth66667 might have caused it.  After upgrade network core switch (Catalyst 6513) where RP is at to 12.2(33)SXI5, the issue was gone.