What is the background to your menu before you add the audio/video? It all dissappears?
When the video motion in a menu is shorter than the menu duration length (here set by the long audio), it should display the last frame of the movie until it loops. You may have to burn a disk and play on a DVD player to see this.
See "About menu display time and looping":
If your video background asset is 40 seconds, and the loop point is at 10 seconds, the video will end at its end, which will be at 30 seconds once the menu loops.