I had the problem that Empathy would disconnect almost immediately when trying to make an audio call. I started it on the command-line and got the following output:
(empathy:12628): tp-fs-WARNING **: stream 1 0x2614a70 (audio) get_all_properties_cb: Error creating session: Could not create the rtp muxer element
(empathy:12628): tp-fs-DEBUG: stream 1 0x2614a70 (audio) close: close requested by connection manager
(empathy:12628): tp-fs-DEBUG: tf_channel_dispose
(empathy:12628): tp-fs-DEBUG: _tf_session_disposeThe solution was simple but also weird: remove the .gstreamer-0.10 directory from your home directory (so ~/.gstreamer-0.10), after that no more problems with the rtp element being created.