After dealing with the question many times on both IRC and here in the forums, I wrote up a first edition of what I hope will be the canonical reference answer to the question
“How can I get sound from my (browser|voice-over-IP-app|messaging-app) while Ardour is running?”
You can read it here: http://ardour.org/jack-n-pulse.html
I wil incorporate any updates and suggestions into future revisions. If someone would care to fill out the JACK2 section for me, even better.