A lot of good stuff goes out of print it seems. What's discouraging is that while improving technology should make it possible to keep works "alive" forever, the prodocers of these works often seem to have little interest in doing so.
A couple decades ago, publishers would print books in runs of 10,000 to 100,000; unless a book sold well enough to justify keeping the plates in storage, printing even one more copy of the book at a later date would be a major undertaking. Likewise with other media.
Now facilities exist to make publication-on-demand reasonably practical. To be sure, mass-production is still (and probably always will be) cheaper than one-off, but one-off price and quality are becoming quite reasonable.
Why, then, are there so many works which are effectively unavailable? It would seem like it should be a fairly simple matter for publishers to set up a system which would make on-demand copies of just about any of their works; the authenticity of such copies could be assured by marking them with a hologram or other such hard-to-counterfeit feature. That way the publisher would get paid, customers would have the books or music they want, and everybody would be happy.
BTW, speaking of "out of print" music, have you ever heard of the Cambridge Buskers?