http://phpfi.com/79845
This gets called whenever the spell calculating functions have access to the caster, and works in any situation ive tested sofar.
it returns a 10 (1.0 multiplier) if you are a) not a bard or b) have a modifier under 10(no instrument wielded)
I know that in live there are now spells that boost your instrument mod, but I dont have firsthand knowledge of how they work, so if someone wants to add to this to check to make sure the higher level stuff is working, feel free.