New keyboard shortcut, offscreen text adjustment

Using F9 instead of F8 for rate adjustment due to screen reader
conflict.

Changed wording of offscreen text to read "audio narration" instead of
"media overlay", to make it more user-friendly.

Hide offscreen text when MO buttons are not active.

If volume goes down to mute, and user hits unmute, volume restores to a
low but audible level.