You need a more complicated setup to do that. You need to start jack, start Ardour so it uses the jack backend, then load the pulseaudio-sink module (I may have that backwards, I forget whether pulseaudio-sink or pulseaudio-source is needed for playback), and then your browser can use pulse for audio, and pulse will be routed to jack. You may need to check connections to be sure that the pulse module gets connected to the outputs you have routed to your speakers.
You don’t mention which distribution you are using. Here is some info for Fedora: https://docs.fedoraproject.org/en-US/Fedora/18/html/Musicians_Guide/sect-Musicians_Guide-Integrating_PulseAudio_with_JACK.html
or Ubuntu:
Forgot to mention that depending on which distribution you are using, all that might happen automatically when you start jack (especially with the dbus variant of jackd). Check documentation for your particular linux distribution, and check out the jack and pulseaudio documentation. FAQ for jack should be at jackaudio.org, although there was some website maintenance going on recently. Hopefully all the pages are still accessible.
ccaudle is correct, but many distributions will automatically do this as long as you have the package jackdbus installed (it is a seperate package in Fedora, but I think it’s included with jack in Debian) . If you don’t already have it installed, it will. probably fix your problems with shared audio by installing it. You can check by using from a command line, #ls -l /usr/bin/jackdbus or #ls -l /usr/local/bin/jackdbus
You may also need to install the pulseaudio jack plugin