I'm not sure if it's expected or not the volume slider visibility behavior is a little bit odd. When the video is playing and you pass the mouse over the sound icon inside the control box the volume slider shows up. However it never hides itself again, even if the media button is pressed or the control box is hidden. My suggestion would be to close the slider when the button is clicked or when the control box is hidden.
Created attachment 208545 [details] Patch
Comment on attachment 208545 [details] Patch OK, makeTransparent sucks as a name for this IMO, but I guess it's what we're stuck with.
Comment on attachment 208545 [details] Patch Clearing flags on attachment: 208545 Committed r154000: <http://trac.webkit.org/changeset/154000>
All reviewed patches have been landed. Closing bug.