I can’t help but thinking that this may be a potential use case for microformats. I notice there’s already some useful pages and research on music and even sheet music on their website.
If nothing else, I’d recommend that you or others delving into the process of looking at music metadata try to emulate the process behind what microformats are and how they work. I think it’s highly useful to take an overview of what and how people are already doing things in real life situations, figure out common patterns, and then documenting them to make the overall scope of work potentially smaller as well as to indicate a best path forward. Many companies will have created proprietary formats and methods which are likely to be highly incompatible or described, but not actually implemented in actual practice. (Hint: avoid unimplemented suggestions at all costs.) Your small polling sample already indicates a lot of variability, and I suspect your poll is very biased give people who would most likely be following your account.
A good starting point for answering your problem might be to do a bit of reading on microformats and then asking questions in the microformat community’s online chat. I suspect there are several people in the community who have done large-scale work on the web and categorization who might be able to help you out as well as point you in the direction of prior art and others who are working on these problems.
If you need help in understanding some of the microformats material, I’m happy to help you out via phone or online video chat and introduce you to some folks in the area.