How can I set Totem to show the visualization instead of the album art when I am playing a sound track?
As far as I can tell, when embedded cover art is available, Totem will always use it. There doesn't seem to be a preference option to force it to use the visualization instead.
This came up on Ubuntu Brainstorm, and the person who mentioned it was encouraged to open a bug report. It doesn't look like this ever happened -- at least I couldn't find any mention of the problem in Totem's bug tracker -- so I'd encourage you to report it.