Enabling mulicast on a PowerConnect 3548P VoIP switch (in stacking mode)

I have 4 3548P switches, running in stacking mode, as the endpoint backbone for a VoIP phone switch network.

How do I enable multicasting on the stack so that when the phones initialize and register with the main VoIP server, which acts as the VoIP network's DHCP server and tftp server to deliver the endpoint phones their firmware load and configuration data, the backplane of the network doesn't get bombarded with broadcast packets but rather has the switch fabric smartly use IGMP to multicast switch the firmware and configuration delivery packets from the main VoIP server to the endpoint phones?