OR they can just put all Frank Sinatra's songs in the game
ALSO put the first song you here in fallout 1....... that one that goes "a gift i never dreamed of, in my imagination" that's all i remember.....
Sinatra's works are still under copyright protection, so they'd have to pay royalties. However "a gift...(actually called A kiss to build a dream on) by Louis Armstrong(and all his works, along with the other works of classic big-band artists) are all in public domain, much like the music that was in GNR, and the music in the mods that are part of A Secret Stash, and More where that came from by OldManMose76 are also public domain.
The real problem is, even when things are public domain. If you're putting them in something that's for-profit you still may need to get permission to do it. However doing it for non-profit, public exhibition, or for private individual use-packaged, is perfectly fine.
So the people complaining about the lack of music?
Step 1: Complain about convoluted copyright laws, and the amazing 150yr copyrights on crap
Step 2: If you're playing on a console, you may want to consider step 3. Because neither MS nor Sony will ever allow modded content on their consoles.
Step 3: Play on a PC, and enjoy customized, modded content.