Someone took
Gouki's theme and
Gouken's theme from Street Fighter IV and
mashed them together. It works rather well, although it kind of breaks down toward the end, due to an extra couple seconds of downtime in Gouken's theme that throws off the synchronization of the two.
While I'm at it, here's the rest of the character themes:
Ryu's themeRyu vs Gouki themeRyu vs Gouken themeKen's themeChun Li's themeGuile's themeZangief's themeSagat's themeM. Bison's themeCammy's themeSakura's themeRose's theme And the new guys:
Crimson Viper's themeAbel's themeEl Fuerte's themeRufus's themeSeth's theme 1Seth's theme 2 Normally, these themes don't play during usual fights. Each level, I think, has its own normal music associated with it (and which I'm not going to bother to hunt up and post here). From what I can tell, though I may be wrong, the actual character themes only play during the Rival battle in arcade mode, when you're fighting against (or sometimes as) the above characters. Sadly and annoyingly, several of the characters don't have their themes in the game at all, aside from small snippets during their anime intros and endings. These are E. Honda, Blanka, Dhalsim, Balrog, Vega, Dan, Gen, and Fei Long (Rufus's theme is not Fei Long's theme, despite the several videos on Youtube saying that it is). Maybe they'll become available later via DLC or something, I dunno. I'd probably get them if they were, assuming the prices weren't too high.
Also, here's my personal favorite from the game overall, because it's a medley of several of the above themes (Ryu -> Ken -> Viper -> Rufus -> El Fuerte -> Abel -> Guile -> Chun Li -> Ryu again -> Gouki -> Gouken):
End Credit theme. (Just wish I could have found a version on Youtube that doesn't have the cheesy speech marring the music.)