I briefly worked at Microsoft xbox back compat where we made the Xbox OG and Xbox 360 games work on Xbox one and newer generations and I know the PMs spent considerable time and effort doing similar things for the earlier Xbox games to be allowed for us to have them work on newer Xbox generations. It's really surprising that sometimes IP changes hands enough in some cases the owners might not have even known they owned something. I think the legal and permission always definitely one of the harder problems. I know music in some games were also particularly challenging, a few games had to have sound tracks removed or replaced.
A lot of contracts are for a specific release, and a lot of IP gets contracted out (music, art, sometimes even game engines). Especially for music, it's not uncommon for the license to be very specific: you can use our music for this particular game, on this particular console, released in this particular region.
