Okay, awesome, I didn’t

Okay, awesome, I didn’t notice the pitch modulation during the beta (although I played on the soundless laptop more then once).
Non repetitive is key for sure.
As for procedural music, I’d offer help if I had more XNA experience, since I’d not mind learning about that area, but oh well

speaking of procedural…

check out 16x16’s amazing game Rescue the Beagles. I dont know if the music is procedural but the rest of the game is:
 
also, check out Dyson (by our friend Alex) it’s fantastic:
 

Or indeed many of the other

Or indeed many of the other wonderful games in TIGSource’s Procedural Games Compo.
MMORPG Tycoon was a conceptual fave of mine.
As for music, you could have 4 (or more) separate audio tracks with increasing beats per minute. The audio mixes in the extra tracks, adding layers to the music depending how frenetic the action is (dependant on kill frequency, or total held points in play). You could also have different fill phrases that activate for when the action spikes or suddenly hits a lull so that any major change in temp isn’t too jarring.
So play starts with a slow pulsing thrumb, punctuated with low-key fanfares (a drumroll-splash maybe) for the first kills. If someone is on a roll then a stocatto drum beat is layered over the top and the fanfares have a little more flourish to them. Next level would be some synth and lastly a dirty sounding guitar for the top level. The fills also bridge jumps over 2 or more levels and the odd special one for multi-kills and when a powerful player gets taken out.
Not procedural but it is dynamic.