I put together videos for songs that don't have one. I usually use clips from the artist's other videos and try to use clips where they are not singing (I'd rather have no lip synching than a mis-match) I also usually try to use only videos that were out at the time of the song's release to make the finished video more time authentic.
I also sync studio versions of songs with no official video over live performances, slowing or speeding the video to synch it to the song. I do it just to put a full version of the song with an appropriately matching video onto YouTube and not in anyway to detract from the artist singing live (although I make no apologies for drowning out irritating audience screeching on some)