I posted a mini-review/report on the film in alt.music.sonic-youth -- I'll reprint the pertinent details here:

I'll skip to the closing credits first -- SY are credited above everybody else as providing the score. Following this is a list of all the songs in the film, the first being "Theme From Things Behind The Sun", credited to all 5 members and "performed by Sonic Youth". This track plays at 3 points in the movie, during the intro, with vocals by Kim, again about 40 minutes in, instrumentally, and as the final song in the credits, instrumentally but uninterrupted.

After this comes "Stranger", also credited to all 5 members and "performed by Sonic Youth". This song is played briefly, overlapped with another song performed by the lead character's band in the film (a band which, incidentally, includes J. Mascis on drums and the guys from Redd Kross, all of whom have speaking roles).

The only other song credited to Kim/Lee/Thurston/Steve/Jim is curious... it's titled "Magic Air" but the performance credit goes to "Low Priority". I just checked sonicyouth.com past news, and have found my answer:

"The week before Xmas Sonic Youth completed work on music for the soundtrack to a new movie by Allison Anders titled "Things Behind the Sun". You may know some of her other movies like "Gas, Food, Lodging" and "Mi Vida Loca". We covered a bunch of musical territory, from some out there jams to a few tasty acoustic numbers. Also, a mysterious boogie group called "Low Priority" weighed in with warped southern workout which was frighteningly accurate."

Otherwise, the only SY music in the film is uncredited incidental sound, most of which takes place in gruesome flashback scenes... this ranges from strumming behind the bridge (mixed in w/ other non-SY score material) to washing overtones to an unmistakeable Kim guitar jam. There is one extended piece that I'm pretty sure is SY -- if it's not, they do a good imitation.