Eternal Sunshine's soundtrack was composed primarily by Jon Brion: composer, producer, singer, songwriter, musician, and all aroud quintuple threat of the musical world.
Brion was in various bands like The Bats, played solo gigs, and even co-produced Kanye's Late Registration (check out this article for more info on how Brion's score for Eternal Sunshine impacted rap music and the music industry as a whole).
In terms of his film career, Brion has composed for Punch Drunk Love, I _ Huckabees, and Magnolia (which won him one of his two Original Soundtrack Grammys, along with Eternal Sunshine). Besides Brion, the soundtrack features songs from The Willowz, The Polyphonic Spree, Don Nelson, and a feature from Beck.
Speaking first of the un-original songs selected for the film, we can see a very unique motif connecting them all: sunshine. We're not talking about the sunny sound of the instrumentals; this is a lyrical connection, something which the primarily lyric-less genre of film soundtracks rarely use.
But if we listen to The Polyphonic Spree's "Section 2" and "Light and Day," Willowz "Something," and even the trailer song that didn't find its way into the film, Electric Light Orchestra's "Mr. Blue Sky," we can hear that each one of these songs has a bright sunny spin that isn't just contained to the uplifting instrumentals.
But let's not forget about what could be considered the film's emotional centerpiece, Beck's cover of the The Korgi's "Everybody's Got to Learn Sometime." It's another sunny song as Beck sings, "and I need your lovin' like the sunshine," but it's certainly not upbeat like the rest of the tracks.
In fact, the song is focused much more on the constant repetition of the title that speaks to listeners as a solemn reality check…and perhaps even an admonishment against the characters' choice of Lacuna over learning.
As a whole, Brion's score has much more in common with this Beck cover than the other happy tracks. The movie is about memory and loss and regret and the emotional mood of the score reflects this. Tracks like "Phone Call," and "Down the Drain," give us this feeling of an empty sadness meant to mirror Joel's journey through the destruction of parts of himself he holds dear.
But sometimes this destruction gets a little more intense. The film is part science fiction after all, and the eerie foreboding of something being not quite right is felt through tracks like "Main Title," and especially "A Dream Upon Waking," which we hear when Joel first begins to realize he's in his own mind as his memories are being erased.
It's moments like this where the usually subdued score comes to life, just as Joel's awareness and the urgency it brings compels him to fight against the dreariness and get back to those sunshiney moments.