Q: Play a video as audio only?

You may need to change the file type to "Music Video" (using iTunes), then your movie file will be displayed under "Music" menu on iPod. And only music is played without video.

User manual is helpful ;
http://manuals.info.apple.com/en/FifthGen_iPod_FeaturesGuide.pdf

The folder you created to put the video (if music video) will show up in the "normal" playlist on your Ipod. It will play there (audio only) but still needs to load the complete (large) file. This will work automatically, especially with "Music Videos" .....

Just access the video from your Music menu