Well, first, the rules probably have changed since that boxed set was produced, and second, that set probably consists of dozens of copyrights. Most recently the maximum number of pieces that can be contained in a collection has been explicitly defined. And, with a boxed set, like a "best of" or "Decca years" or whatever, almost certainly the individual works had their own copyrights, especially if they were a compilation of previously released/published material. It's even possible the boxed set only was copyrighting the new artwork and the compilation as a work, and not the individual pieces in it, which probably have a stack of copyright registrations associated with each one. Publishers have the lawyers and money to do those sorts of things, and are not trying to save money by batching a bunch of stuff together that their lawyers will tell them needs to be done otherwise.
As of 2019, "Group Registration of Unpublished Works" is the new way to do what it sounds like you want, and it is limited to electronic/digital file submission. I suppose you could knit together groups of small files into a single one and name them Suite A, B, C, etc. to try and save money, but if you need a DVD to submit MP3s, you're almost certainly going to need multiple submissions. It might be worth doing some triage on your recordings.
Information regarding Group Registration of Unpublished Works (GRUW)
copyright.gov
Or, you could hire a lawyer that makes their living doing this stuff to guide you through this process!