Use-case for audio/video track mixing with ducking effect

The MediaStreams Processing document had a scenario which I think isn't
covered by the existing use-cases in the wiki: "Play video with processing
effects mixing in out-of-band audio tracks (in sync) (e.g. mixing in an
audio commentary with audio ducking)"

A very common example of this is DVD commentary tracks. When there's audio
on the commentary track, the main track volume is reduced --- preferably
gradually over an interval *before* the audio on the commentary track
starts, so the ducking filter necessarily has high latency. Yet you need
both tracks to stay synchronized with the video. You also want to be able
to pause and resume the combined video+audio without disruption. This is
quite challenging.

A browser-oriented use-case could be: "User wants to play a video from a
Web site, with a third-party commentary track downloaded from another site."

Here's a demo of this in action with ProcessedMediaStream:
http://people.mozilla.org/~roc/stream-demos/video-with-extra-track-and-effect.html

Rob
-- 
“You have heard that it was said, ‘Love your neighbor and hate your enemy.’
But I tell you, love your enemies and pray for those who persecute you,
that you may be children of your Father in heaven. ... If you love those
who love you, what reward will you get? Are not even the tax collectors
doing that? And if you greet only your own people, what are you doing more
than others?" [Matthew 5:43-47]