Left 4 Dead's main tune was a sorrow filled piece, that made you realize that many have died from the Zombie Apocalypse, wondering if you will be the exception. At the end, when the chopper/APC/boat arrives, the music seems to be shouting "COME ON! YOU CAN DO IT! SHOW THESE ZOMBIES!". Your teammates are being attacked by hunters, barfed on by boomers and thrown across the map by tanks, but you are still getting to the vehicle. The AI director is throwing everything it has at you, but you keep running. Once you get to the vehicle, you feel like you really accomplished something. L4D2's main tune sounds like something, really, really horrible happened in the last 5 seconds, and this is the aftermath.
The jukebox in L4D2 not only plays "Re: Your Brains" by Jonathan Coulton, it also spawns a wave of zombies on you. Paranoia Fuel, anyone? Bear in mind, it's not that the song spawns zombies, it's when the song spawns zombies:
I think I speak for all of us when I say I understand
Why you folks might hesitate to submit to our demands...
But here's an FYI:
You're all gonna die screaming! (cue zombie swarm)
L4D2's version of the Skin on Our Teeth music mentioned above. The second that badass track starts you feel like if you don't haul ass NOW some completely gruesome and epic thing will happen to you, as opposed to L4D1's version that seems to go: "You can do it!"
The first version tells you that you can escape. The second version tells you that if you don't get out of there now, you are dead. And there are several hundred zombies in your way.
Ladies and gentlemen, the song from the Zombie Survival Guide trailer for Left For Dead 2: Chocolate Helicopter.
The Parish's Horde Theme. The frantic trumpet makes it sounds like you're in a surprisingly fun mad dash as you slash and shoot your way through — exactly what most Horde encounters are. And in the Parish, the final level is a bridge filled with zombies, fun mad dash indeed.
While it's completely different from every other song, Save me some Sugar from The Passing is a nice love song... or not, as it's a Breakup Song - specifically, one belonging to the category of "I Have To Go/It's Me Not You". No wonder the Witch Bride gets really angry if you play on her wedding day.
Even though it's less than ten seconds long, The Monsters Without is a perfect song for reaching a safe room, especially if your team - or whoever's left on it - barely manages to limp their way inside ahead of the zombies. L4D2's version is equally as awesome.
The pre-level saferoom music is a subdued piece of awesome music, giving a safe, relaxed mood mixed with feelings of preparation and anticipation of the zombie-filled path outside. In Left 4 Dead, it's slightly different for each campaign; in Left 4 Dead 2, it isn't - with the exception of The Passing, in which it's a mix of both games' songs.