During my day, I play Banshee audio of my local music collection. When a friend sends me a funny YouTube video to watch, I usually do the following:
1. Click the link (most of the time I don't realize it's a YouTube video, but even if I did I would probably click the link first before remembering my audio problem)
2. The video starts and I realize that playing two audio sources makes both of them kind of useless
3. Pause Banshee (usually with a Do shortcut, but occasionally by going back to the app and pushing the space bar)
4. Go back to Firefox, skip to the beginning of the clip and hit play
5. When the clip is done, I usually forget for a while to turn Banshee back on, but when the silence is noticeable, I go back to Banshee and turn it back on.
This whole thing is really frustrating, and it's the same with just about any "I'm playing some audio, but now I want this to play" thing.
(Note: satisfaction / efficiency tags added according to mpt's "What do you mean by "usability" found here:
http://mpt.net.nz/archive/2008/08/11/usability )