The publisher (ISBN holder)is the person who gets contacted for copies, but it's easy enough to re-direct to author(s).
Yes, for a book with music in it an ISMN is needed: if it's a score, but with little or no text then an ISBN is not needed. If it is music and comment / text, both are required.

Not having either will reduce sales: it also infringes law (I think - UK). Having an ISBN obliges you to send 5 copyright copies to the copyright libraries via an address in London. The British Library will also want one. So add the cost of 6 freebies to your cost-per-unit.

Good luck.

Paws