Yes, I’m using the term “app” instead of ye olde “application” now, just to sound cool.
Chrome recently introduced a feature, which allows you to see which tab makes noise. This is useful if you have a lot of tabs open, and a Flash ad runs berzerk, so it will ruin your music or just annoy you to hell.
Did you know, that Windows has the same feature, just on application-level?
Since Vista, you can click on the speaker button in the “System Tray”*, then choose “Mixer” (in my german Windows).
Then you can see, that currently Skype uses sound, for calling the famous Echo service.
* This term is wrong, it’s actually the Notification Area, but people seem to know it by this name.