As a "muso" myself, all I ever take on board is a baton
I am not under the impression that there is an exemption per se... Although most musicians will call ahead to make sure that it will be ok to take their instrument on board. In 95% of cases there is no problem as long as it has been cleared beforehand.
For larger instruments, it is not at all uncommon for a musician to also book a seat for themselves and "Mr. Cello" or "Mr. Bass" (I kid you not...)
I think the thing to do is call the airport / airline in advance and make sure you get the name of the person you speak to.
Cheers,
Martin