Hampshire actually listed several in post #12, though you may not have seen those films, or remember the heavy use of songs by one artist:
Harold and Maude* lists nine songs by Cat Stevens on its soundtrack.
The Graduate lists fourteen songs by Simon and Garfunkel on its soundtrack.
Batman lists six songs by Prince on its soundtrack.
And, as snfaulkner notes, Queen had already done the Flash Gordon soundtrack, too.
A few others that come to my mind:
Xanadu has five songs by Electric Light Orchestra, including a duet with Olivia Newton-John (who also performed on six songs, though she was the movie’s star; ELO never actually appears in the film).
The Magic Christian has three songs by Badfinger.
Tron: Legacy’s soundtrack was done by Daft Punk.