Maria:  I doubt that most musical organizations are into double-entry bookkeeping!  But if I were to make a guess, it would count as an expense.  Admittedly a large one.  (a) Sheet music is designed to be used up (per the Fair Use Guidelines).  (b) Used sheet music has a value of approximately zero.  Ask any insurance agent, but preferably check with a CPA.  (c) Many orchestras rent a majority of their music, thereby making it a straightforward expense.  Our opinions are meaningless.

All the best,

John