Browser idea: each tab should have it's own volume control. I should be able to hit some hotkey or button, and then scroll-wheel to the desired volume. It's annoying when I have a browser-based music player, and then I open up a noisy page like this one. I want a standardized way to mute or turn down this page's volume without having to hunt for a custom button.
I really liked Windows 7's per-app volume mixer [0] (Windows 7 is the last Windows I really used, not sure if they kept it or not in later versions).
I'm sad that the idea hasn't made it to other operating systems, with a system level API to allow for setting volumes independently on sound streams in different apps (an app could expose any arbitrary number of sound streams - most apps would likely have just 1, but browsers could have one per tab).
XFCE 4.10 does at least launch pavucontrol when you click on the "Sound settings..." menu item under the volume applet. However, you're right that it only shows one volume slider in the applet menu itself.
Per app volume control arrived in Vista. Previously you had a single master volume control in the notification area which if you double clicked would open the sound mixer control panel.
That panel on XP (and before) only permitted per-device volume control.
For reference see [1] at around 04:00 and Larry Osterman's posts from September and December 2005 [2][3].
JACK is a huge amount more than just a per-app volume mixer, it's basically a virtual audio cabling system. Very handy if you've ever had a need to pipe audio between applications or manipulate audio streams from arbitrary apps.
It's also a complete pain in the arse to get running on Linux, due to the ALSA+Pulse stack being in the way.
If your goal is to use Linux for audio production (or some other use case where JACK would be necessary), it helps to use a distro like Musix or Ubuntu Studio that already takes care of that "get running" part for you.
JACK is pretty complex, but infinitely useful; one of those things I end up installing long before I decide I actually need it.
