disksrc -> mp3parse -> mpg123 -> audiosink

disksrc reads last 4K chunk from disk.  Sets EOS on the buffer, pushes buffer out to peer.
disksrc:src sets itself to EOS and chains to mp3parse.

 mp3parse takes the buffer and slices it up into several.
 Each buffer gets chained off to mpg123.

  mpg123 decoders each frame and sends it to audiosink.

   audiosink dumps the audio data to the sound card.

  mpg123 returns from the chain, and returns.

 mp3parse returns from each of the several chains,a nd returns.

disksrc:src returns from the chain, signals EOS, and sets disksrc's EOS flag

At this point the pipeline state code takes over and starts to set everyone to READY
