It seems like there might be more than a PID that could help you support creating metadata for the 2k scores. I have broken down what could be helpful into a couple of categories. Happy to talk further if it would be helpful!
On PIDs and music:
There is no DOI for music, though I believe there has been some very preliminary discussion about it in music circles. That means in 5 years or more they may have decided on something :). I think a standard DOI would be sufficient for a digital musical object at present.
The ISMN could work: https://www.ismn-international.org/howtoget
On metadata and credit and music metadata in particular:
But, it sounds like there is a conflict between the asset management system and the content type this person is trying to document. Music cataloging is certainly an option for recording material, using LCSH, LCNAF, LCGFT and MARC 21, but that does not usually mix easily with a DAM or archival system.
I think schema.org could be helpful, especially in a Dublin Core context: https://schema.org/docs/search_results.html?q=music