This API is still being discussed in Pulseaudio mailing list and when it is ready, we should discuss how to adapt it at GStreamer level and apply a patch in WebKit to use it. Ideally, after that we should be able to revert the fix for bug 118974.
*** Bug 145609 has been marked as a duplicate of this bug. ***
Closing this since the PulseAudio developers never completed their browsers API and all major distros except Debian have agreed to turn off flat volumes.