O wiki do OpenWrt tinha uma resposta:
With IGMP snooping, multicast forwarding is disabled for bridges. One pure bridge solution is to disable multicast_snooping.
Add the following in /etc/rc.local
echo "0" > /sys/devices/virtual/net/br-lan/bridge/multicast_snooping
Replace br-lan with your actual bridge interface, sometimes also called br0.
This will forward all multicast packets to all ports on your bridge, making igmpproxy or udpxy unnecessary. In large networks, this may not be desirable.
Lembre-se também de que o multicast por Wi-Fi é um matador de desempenho.