(as per RFC queue'd draft-ietf-avt-rtp-vorbis-09).
Not tested against other implementations, and some pretty bad PTS
issues. RTP only provides us with a valid PTS for the first frame in a
packet, and we don't know the length of each frame, so, hmm... I wonder
how to compute the PTS for non-first frames...

Also, we would need either an SDP parser to use Vorbis (properly) as a
dynamic payload type, or to use a proprietary static payload type and a
proprietary static RTP clock frequency.