I don’t really know many alternatives, but I’ve heard something about slimserver.
A quick google led me to the following site: http://www.slimdevices.com/pi_features.html
Supposedly it does what you wanted:
– Web Interface—control the player and manage playlists from a web browser
– Supports multiple synchronized players
It used to be the free open source slimserver. Now it’s been taken over by Logitech to feed their overpriced music clients with music.
I think it has the same idea of firefly and a soundbridge, but the software plays 1 song which is sent to all clients, which firefly doesn’t (can’t?) do.
But you’re not the first person I’ve seen that misunderstood that.
Maybe an idea for a feature to switch between those types of functionality..