This mockup by queckjap is both cute and fantastically representative for what the sound menu does with Rhythmbox.
Of course there are issues with this.
Firstly the sound menu, even when playing music, still controls other applications. Turning the icon into a note could confuse people into thinking that it is solely for controlling the volume in of Rhythmbox and not system-wide. (e.g. muting the ‘note’ thinking they are muting Rhythmbox, cue panic when YouTube doesn’t play sound…).
Secondly, as more applications begin using the Sound Menu – of which many running in unison – who decides what icon gets priority?
All that said the idea is still great to look at and something I wouldn’t kick up a fuss about seeing implemented.