Instead we get:
Error: unknown or invalid packet type 'sound-control' received from WebSocket(ws socket: 127.0.0.1:10000 <- 127.0.0.1:36802)
Minimal fix in r25611.
Still TODO: disable the button if audio forwarding is not available.
Better fix in r25612.
Refactoring in 25613.
Backport in r25614.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2636