There are a lot of technical limitations for something like this. Flash is controlled by the browser, and most media embeds are flash; an extension would need

There are a lot of technical limitations for something like this. Flash is controlled by the browser, and most media embeds are flash; an extension would need to prevent the browser from killing the flash element on page away, which I am not entirely certain if it's technically possible in Chrome or Firefox without extensive modifications to the browser itself. If you're referring to embedded media using HTML5 tags, then I'm pretty sure you could write a simple Firefox Jetpack script to return null inside the page, and play it in the status bar instead. But that wouldn't work everywhere (as few websites use the audio tag). Another alternative, which I doubt will gain adoption, is using a custom protocol like media:[url], which when you hit play, instead of playing it as a page element, it tells the browser to play it (like Windows Media Player or VLC). The only problem would be, of course, getting the browsers and web developers to adopt this scheme.