I have a dedicated place for all my "non captured" Digital Assets. I used to call it "Non-Tape Items" but that's no longer appropriate.
I also copy them to the event as needed as they usually aren't very large.
This way if the Event is moved or archived then everything goes with it.
Once the job is complete I can delete/archive the Event but still have the digital assets for use if I do more work for that client or the files get repurposed into Disc Sleeves, Web Pages, Blogs or Podcasts.
I have this folder also backed up by TimeMachine as well as archived onto a separate external drive.
As for music, this is why I know almost exclusively use online services like "PlayProductionMusic" it allows me to create project databases of the music I chose for each job. I've got every edit I've ever done going right back to 2006. I refer to it often.
Of course if your music is commercial or composed you will need to archive it.