In my adventures on another brand of music servers and UniFi switches, I also experienced odd behaviors when dealing with snooping. That items showed up when you turned snooping off is somewhat not surprising. Without a IGMP Querier, snooping is only half done and you get odd behaviors. An IGMP Querier works hand in hand with IGMP snooping to form a complete configuration. My solution on the above troubles was to ditch our UniFi switches and get Meraki switches. I normally specify Meraki, but the customer had rejected them due to price. Big mistake by all. What I would recommend trying is the set up below, which is a fully set up network for multicast traffic. So first reenable IGMP snooping. Second go to one of your Meraki switches and create a Layer 3 interface (Routing and DHCP). The chosen switch should be the one most central to the music streams. Preferably where the server is. Configure as the screen shot below using your subnet, not mine. Just choose an address on your existing VLAN subnet or on the VLAN subnet that has the music on it. Choose Enable IGMP Querier. The Querier works with Snooping to calm down the network and stop packets from going to ports that don't need them. Without it some multicast systems are just not going to work. You only want one Querier enabled switch on the subnet. Do not enable it on another switch or problems will arise. My music system also required QoS on the switches to prevent pauses and drops. But I think the most important thing is getting the multicast snooping working right. It seemed Linn did not even suggest QoS according to the documents you shared. If enabling a Querier does not work, then you can always undo it, but I think it's worth a try. Enabling that L3 interface is not going to turn your switch into a router. It will just be a L2 switch with an L3 interface that is only used for a Querier.
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