How about Rockler part number 52233, "Power Twist Plus Link V-Belt
style belts".  I have not used this belt on any musical instruments
but have it on three drill presses, a band saw, a stationary belt
sander and a table saw.

I found that if a regular V-belt sits unused for a period of days
to weeks, then the belt would "take a set" around the pulleys and
there would be a lot of vibration for quite a while when I used the
tool again.  With the link style of belt the belt vibration totally
disappears in just a minute or so.

Also you can make any length belt you want.  If it is a little too
long or short just take out or add a link as needed.  I have not tried
it on multiple belt drives but I think it would probably work well.
Their advertising says it is ideal for multiple belt drives (for what
that is worth).

My local industrial bearing supply company stocks it in a couple of
widths.  Grainger calls it "link V belt".  McMaster Carr calls it
"adjustable length V belt".  Both Grainger and McMaster Carr have
it in several widths.

John Dewey