I guess this answers the question I was having about sharing music between subnets.
I’ve got 3 subnets routed together with Linksys routers configured as routers rather than gateways. All nodes on the network can see each other, but I guess Firefly just won’t pass between them.
It’s not a huge deal since one network is for my family (firefly’s on that one), one’s for testing, breaking and working on other people’s computers, and the third is an outer peripheral subnet so I can have the two routers behind it routing instead of using NAT.