To delete the sound that comes with a theme:
Make sure the menu whose theme music you want to delete is showing in the iDVD window.
With your pointer over the menu, and with no buttons or text objects selected, press Command-I to open the Menu Info window.
Drag the audio file icon out of the audio well (shown below) in the Menu Info window.
When you release the mouse button, the file disappears in an animated puff of smoke.
The audio well is now empty. You can leave it empty or drag a different audio file to it.
To delete the theme music from your entire project, repeat this process for each menu in your project.