Originally, the soundtrack was going to be all covers by Elliott Smith, but he died before completing the project. We still get Elliott's excellent versions of Big Star's "Thirteen" and Cat Stevens' "Trouble" (which of course carries heavy Harold & Maude connections), as well as one original "Let's Get Lost" (which appeared on his posthumous album From a Basement on the Hill).
Director Mike Mills (not the bass player for REM) then hired Tim DeLaughter (formerly of Tripping Daisy) to create an original soundtrack with his symphonic pop choir The Polyphonic Spree. Those songs really keep the emotions being played with in the film simmering all the time, and their ethereal sunshineyness works amazingly well with Elliott Smith's passionate, haunting performances.