Flash is one of the most popular browser plug-ins these days, not only for design, but for rendering music and videos also. Although it brings a lot of new features and interactivity, there are times when you just don't want to be disturbed by it. A classic case is when you open a new page in the background and you're immediately bombarded with loud adds or music. Not good it you have more than a few tabs opened and have no clue which one started all that noise. And don't forget the videos that play automatically on the YouTube pages!
Unfortunately, there's no easy way to control the sound output from Flash, either from the browser or from the plug-in itself. I have come across this problem a few years back, when I wanted to play some browser-based Flash games and listen to music at the same time. The games had no mute button... and so I began searching the net for a solution.