The reason you can't have music on a layered menu is that it's not a single
menu. The DVD spec doesn't support layered menus, so DVDSP does it by making
a series of nearly identical menus - one for each layer.
So every time you highlight a different button, you are actually jumping to
a different menu. If you had music on the menu, it would go back to the
beginning every time you highlighted a different button! So they just
disable that function.
Layered menus can also be very slow to navigate on some players, because of
the time taken to load a different menu every time you select a button. They
are best avoided unless you really have to use them.
The really high-end authoring solutions don't have a layered menu option,
since it is really a cheat.