Finally read the whole thing. While you've raised some decent points, they don't always apply to all levels. Especially not in multiplayer. In MP the player doesn't really have much time to take musical choice into consideration, much less let it affect their gameplay, as they're too busy trying to avoid being killed and/or camping above the base being ready to score and lots of other stuff. Most MP levels instead choose a musical track that is upbeat and 'kickass', while still fitting the basic theme of the level (Wicked Wood's music includes the use of flutes and some other tribal elements, while Happy Semiconductor CTF has a highly electronic feel). Personally I completely forgo atmosphere during actual gameplay and instead focus on the paths through a level, at which point music becomes completely irrelevant. During important matches I replace the music with a different song entirely.

Slow and atmospheric music only becomes a real actual point during single-player levels, where the player has the time to stop and take in the sights.