Became my favorite movie after seeing it several times in 75 and 76. Hasn't been dislodged since. The ABC LP and CD captured many of the musical highlights but nowhere near all of them.
I won't pretend that every song is a gem, but the film is packed with great songs and musical interludes. Ronee Blakely's songs are a treasure, Keith Carradine's "I'm Easy" won a very deserved Oscar, and Richard Baskin's arrangements really held it all together. and the closing "It Don't Worry Me" is one for the ages.
It's been awhile since I saw it, but I'm pretty sure there isn't a background score, and most if not all of the songs on the existing LP are taken directly from the film. The only thing you might be missing is a few background country western riffs as source music, usually only heard in brief snippets.
There are many things missing. Without going into excruciating detail--for one, I haven't and won't go through the movie and list things not on the soundtrack--but off the top of my head:
* the live Bill, Tom, and Mary song at the club perhaps called "Since You've Gone" * the college marching band that plays a Barbara Jean song as it greets her at the airport * Ronee Blakely's touching solo rendition of a spiritual in the hospital chapel * there are missing songs from the Grand Ol Opry sequence
Also, a welcome addition would be Ronee Blakely singing "Bluebird" from one of her own albums--in the film Timothy Brown sings it.
And let's not forget that the initial cut of the film, the cut that was famously showed to Pauline Kael, was seven hours long. I'm sure there was a lot of music recorded for this film that never made the finished cut.