I've never seen a game that uses so few musical tracks so much, and they keep getting stuck in my head.  I've counted about 8 in total after playing the game for almost 10 hours now, 4 of which are played constantly, the other 4 only very occasionally.  I've started to make up little names for them in my head, especially the more common ones, and I'd like to share them with you all.  I should note that most of this music plays in cutscenes and more laid back scenarios, so I may be missing music in action sections, but I haven't really noticed any.  I'm also not counting ambient tracks without any kind of melody.  Feel free to add to the list if I'm missing something or contribute your own titles. 
 
One weird thing I've noticed is how frequently the game changes music during cutscenes or just doesn't match the mood.  Occasionally the music swings from one extreme to another.  Here's a few examples on Youtube (the first one is also a comedy gem):
The Infamous FK Scene
Whiplash Inducing Changeup
Wait for the end

TRACKS YOU HEAR ALL THE FUCKING TIME

Lazy Sunday aka. Super Whistleo World
Starts off sounding like someone whistling the Super Mario World theme just slowly and badly enough that it doesn't merit litigation.  Supported by an equally dragass rhythm guitar and a special guest appearance by a trio of kazoos about halfway in.
Played while fishing and during mealtime cutscenes.
 
Fairy of the Forest aka. "Na Na Naaa!"
Twangy guitar with "na na naa"s on backup.  Probably the only vocals in the game (that you'll remember, see rarer tracks).  Sounds like something you'd hear in a shop that deals exclusively in teddy bears, porcelain angels, and magic Indian paintings.
Played primarily during cutscenes in the woods and in the title screen.
 
The Game's Afoot aka. The Soundtrack from Private Inbreastigator
You can't fucking tell me that this doesn't sound like porno music.  It does have an appropriate air of mystery to it, almost like something you'd kind of hear in a private eye flick, but it's too swanky.  Instruments include a trio of a piano, drums, and the pornoiest saxophone I've ever heard.
Played in the "Red Leaf Room" at the start of chapters and during key junctures in the investigation.
 
Five-Oh aka. Generic 80s Buddy Cop Theme Music
More jazz, this time it's got a few more brass instruments, a more pronounced piano, a quicker beat, and an authoritative tone.  Very bombastic, but the endlessly looping enthusiasm wears on the nerves a bit as you drive through Greenvale.
That's right, enjoy it, because it's the only music track that plays while driving.
 
TRACKS YOU HEAR VERY FUCKING RARELY
 
Vision of a Goddess aka. THIS IS YOUR LOVE INTEREST 
I've only heard this play once, and I would have forgotten about it entirely had I not been watching Youtube to double check my mental list.   The main instrument is most likely SNES quality synth, but it could be some kind of organ.  Operatic vocals in the background match the tune of the music, but clash with the artificial sound.
To my knowledge, this only plays when first meeting Deputy Emily Wyatt.
 
I Hear the Voice aka. Is this Endless Ocean? 
If you've played Endless Ocean at all, then you know the kind of grandiose echoing vocals I'm talking about.  Now, subtract the talent of Hayley Westenra and layer that on top of jangly vibrating synth vocals pitched higher than humanly possible and you pretty much have this discordant gem.  This is why humans should never sing with robots.
Again, only plays once to my knowledge, this time in the intro cutscene.  You won't miss it. 
 
The Plot Thickens aka. Torchlight?
Now, this piece to me sounds like the background music from the early stages of Torchlight, maybe it's just a weird coincidence.  Still, by this point, I'm reminded of iconic songs from three other games.  This one is a piano and I think a bass.  It actually works quite well, wish they'd use it more in investigation scenes than porno jazz.
You really only hear this when meeting new characters and the tone is sad or tense.  This also plays during a trailer for the game.
 
Keep on Rollin' aka. Rock Organstravaganza 
A mixture of low-key piano and someone going ape shit on a rock organ.  I actually wish they'd use this one more, maybe as another song to listen to in the car.
Plays occasionally when meeting people in a more humorous context.

I think the "na na na" song is pretty good. Barely heard any of the other songs. 
 

Wonder how much you'd like it when you found out that they use the same 2-3 pieces for most cutscenes despite having a larger set of music to pick from.
 
One thing that I was wrong about was the car music.  Different musical pieces are tied to different cars, so upgrade before the brass band drives you insane.

The repetition almost becomes funny in certain instances.
 
For example, when York enters the car he gives Zach a recap of what they're supposed to do.  I was up late with my brother playing this game over Easter weekend.  York had this one line, "Zach, do you know the best way to find lost children..." which I heard all the frickin time as I wasn't advancing the plot so I could finish the game's most annoying side mission (involves checking in at a bar until a certain character shows up).  It got to the point where we were both just roaring with laughter every time he said it.
 
Now, imagine that, but York's making some offhand comment about people getting horribly murdered to that same knockoff Simon and Garfunkel song you've heard 50 times already.