Just to make sure, all of your links need to be in quotes if they are not. The : in a url can make some yaml parsers think that it is another block, there are other URL safe characters in general that are special characters in yaml so it’s a good idea to put them in quotes.
This is a cool idea! This sounds a lot like what DANTE and AES67 or AVB are used for in pro audio (mixing console sends multichannel and outputs can subscribe to one or more channels), maybe they have some ideas on timing sync which I think would be the hardest part as others have said, it is crazy how small of a jitter your brain can hear.
Have you tried adding 239.255.255.250/32 to your outbound subnets variable? This is the multicast address for SSDP which mDNS ultimately relies on if I remember right, I recall having to do this for Plex in the past.