... is the majority of music I hear on these sites rather illegal then?
simple answer: YES!
I received two 'blue letters'/cease and decist from YouTube, 'cause I replaced the music in live-scenery by parts of same track from my iTunes collection (to get a solid audio track). the YT-filters (machine or Bangladore) identify commercial music fast - esp. when bought in iTS, it seems, it contains a 'watermark', which instantly tells YT "who owns this?"
in terms of legal/illegal: some are tolerant, some not. but YOU don't have any rights to publish any 'found' music (pictures, videos, maps, graphics, buildings, art, ......................)
I tought a seminar on this issue, without being an advocat ... in theory, 99% of YouTube has to be pulled.
OK, I get this 'unhappy' message.
So, nobody will ever know what they are missing , although I did mention all this music and found images on the iMovie's title roll, specifically to respect each one's rights (even as I never got any reaction from the identified owners to my asking them for their permission; no interest obviously...).