The music attached to a film creates the environment, I believe, moreso than the literal environment depicted through the visuals...
Whenever we think of soundtracks, rarely do we think of bands and music artists. Lately, this has changed: The Chemical brothers and Hanna, Underworld and Sunshine, Nine Inch Nails and The Social Network - even Badly Drawn Boy and About a Boy. Randy Newman was ahead of the game early on. So much so, that for me, Randy Newman is primarily a composer. To another generation, Randy Newman is the artist behind 12 studio albums since 1968. His work with Pixar alone has also earned him much praise - as he composed all three Toy Story scores, but additionally Cars, A Bug's Life and Monsters Inc (and currently penned in for Monsters University). So, it goes without saying that he set a precedent with his soundtrack to Toy Story.
2. Strange Things - For me, this is the song the represents this film. When I was younger I was desperate to her this song because it perfectly encapsulates the playful and creative nature of 'play time'. Though the song is used for a montage showing Andy's lack of interest in Woody - it also shows Andy playing with Buzz - and the other toys - in lots of different ways.
4. Andy's Birthday - In a similar way to Strange Things this shows how varied Newmans style is. He plays with lots of different techniques that show his versatility.
3. I'll Go Sailing No More - I had to throw this one in too - light, hopeful but very personal. One great thing about this song (and indeed the soundtrack) is how it makes that personal connection between us human viewers and the toy characters we are expected to relate to. Which we do relate to - because of many reasons, but especially the soundtrack.