More than a red herring, the multicast could be valuable because it obscures who's actually listening. Everyone on the network is receiving these packets so there's no way to single anyone out. Seems like a great move for spy software, 100% plausible deniability. (but I really doubt that's happening here)
My memory is a bit hazy on this one - but I used to run the engineering team for a company that did multicast based IPTV for hotels about ~2003 or so and I'm pretty sure the set top boxes used IGMP to control what video streams were sent to them - all devices on the fibre backbone got all the streams but each device in the rooms (connected by copper) could only handle a single stream.
So multicast doesn't necessarily mean that every device gets every packet... I think.
Also probably not worth using IGMP for audio.... :-)
