When I start my receiver with RCForb Client, I get this message:
Invalid url protocol -> ums://TCP:91.97.219.190:5119/Andreas Home
In RCForb Server; you may need to change the "Audio Type" from disabled to what your using...see screenshot below

That screenshot is old, and doesn't show the new VOIP settings. You may also enable the VOIP server to use the build-in audio support with the client/server.