Here's the themes for every couple. I've included every canon couple (even the ones that have broken up) and three one-sided crushes.
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TylerxLindsay: Hot da Avril Lavigne

IzzyxOwen: Hot N' Cold da Katy Perry

CodyxSierra (one-sided): Snow White Queen da Evanescence (Most of the song is from Cody's point of view. The chorus is from Sierra's point of view.)

CodyxGwen (one-sided): High Above Me da Tal Bachman

CourtneyxDuncan: I Hate Everything About te da Three Days Grace

HaroldxLeShawna: White N' Nerdy da Weird Al Yankovick

HaroldxHeather (one-sided): Just the Girl da The Click Five

TrentxGwen: She Will Be Loved da Maroon 5

BridgettexGeoff: Spaceship da Puddle of Mudd (They DO make-out a lot...)

DuncanxGwen: The Rock mostra da Blink 182

HeatherxAlejandro: Evil Amore from Phineas & Ferb -or- She Hates Me da Puddle of Mudd -or- Womanizer da Britney Spears (Yeah... this couple gets three, LOL. But they all fit so perfectly!)
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If you're confused about why I picked certain songs, just askk me and I'll say why. :)