I did discover an "under the hood" way to change the audio.
Let me start with a disclaimer that you need to be careful when making these sorts of adjustments, and realize that in the process of learning you could possibly trash a menu. I would encourage you to experiment with a backup copy. Perhaps MODIFY a menu, then save it as a new template.
And, I'm not in a position to provide tech support. In other words, you are on your own! But this did work for me and I wanted to share with other potentially frustrated users! :
In the CREATE DISC section, on the MENU PREFERENCES tab, you can look under the designer pane on the right and see a line for "Background Music". Notice where it is pointing. It is ultimately buried in your Documents (or My Documents, depending on your Operating System).
Using Windows Explorer, I navigated to that folder. In this case, it was: "C:\Users\....\Documents\CyberLink\Custom Menus\3.0\Menu_008\Audio"
That folder contained two files:
std_music.xml
Calligraphy.mp3
Calligraphy was the standard background music for the template.
I right-clicked on the "std_music.xml" file and made a copy.
I then copied a new mp3 file into the folder.
I then edited the "std_music.xml" file. It made three references to the "calligraphy.mp3" file. I changed those three references so that they would instead point to the mp3 file I had copied into the folder.
I switched back to PD8 and tried to re-apply that template. It did not work. I eventually learned that you need to switch out of "CREATE DISC" (I just went back to "EDIT"), and then switch back into "CREATE DISC". I reapplied the template, and there was my new background music, saved in the template!