indicator-sound 0.6.6.1-0ubuntu3, Ubuntu 11.04
indicator-sound 12.10.2+13.10.20131011-0ubuntu1, Ubuntu 13.10
In maverick's sound menu, it was possible to activate the banshee or rhythmbox play/pause, back, and forward controls by pressing the mouse on the indicator applet sound icon, dragging to the chosen control with the mouse button held down, and releasing the mouse button. It looks like the sound menu spec at https://wiki.ubuntu.com/SoundMenu is ambiguous as to whether this should activate the controls, but all other menu items in the indicator applet can be activated in this way (as an example, the mute option in the sound menu). The fact that this no longer works for the playback control buttons seems like a bug.
To be clear, click/release on the sound menu icon followed by click/release on the playback buttons works perfectly well. The inconsistency is in the playback buttons being more picky than other menu items, in that they can't be triggered by just a mouse release.
<https://wiki.ubuntu.com/Sound#playback-item>: "Conversely, clicking on a different menu item (or any menu title), moving to a button, and releasing, should activate that button."