Renumbering

I'm trying to renumber all my CD categories in running numbers, and the DVDs separately. Is it possible to renumber single categories? I can number the whole list by categories, starting each one from #1 but that's not what I want.

I'm trying to figure out the order of the numbers in a single category. It's not alphabetical, not quite scan date - is it some hidden definition? Does it depend on recent sorting of different columns?

My sorting in classical CDs is by composer, the other categories by artist.