Chrome 32 added a similar feature, but it works for any site. It shows a small icon next to the title if the tab is playing audio.
I tested this feature and it works for HTML5 video/audio and Flash content. Mute the video or pause it and the Chrome icon disappears.
The sound icon is redundant for YouTube and the other sites I've mentioned, but it's still displayed for consistency.
Chrome shows two other special icons for using the webcam and casting.