If you say "the Movie and Sound bar, near the bottom of the main screen", what are you referring to ?????? When I embed a video and start the slideshow, I just see the video and no movie/sound bar of any kind.
This bar should automatically appear when you selected the video area on your presentation, at least its for me. It residents at the bottom area of the impress editing window, it has a play button, a pause button and a progress bar.
I have a very different problem than yours however. In my case I can play the video with this tool bar, but the video doesn't play in presentation mode, when the tool bar is not available.