Ok, so in the US version:
Some of the music uses different samples. Most notably Ryu's stage, Ken's stage, Zangief's stage, Guile's stage, and Cammy's stage for the worst. I also compared the American and Japanese versions of Blanka and Guile's ending themes each, and the American version seems to sound noisier.
Ken uses a higher-pitched version of Ryu's throw voice, even though he had his own throw voice in the SFC version.
Dhalsim's K.O. cry sounds different, perhaps at a lower pitch?
Fei Long's stage and Ending Credits theme is one pitch higher than the SFC version.
Ryu's throw voice sample was also changed, and the one used in the American version is also played when you pause the game.
The only thing I like about the American SNES version, is that the announcer will say the names of the characters, and even an added "Versus" if you're playing in Tournament Mode. In the SFC version, the announcer always says "You Win" in 2-Player matches, and doesn't say anything at the VS screen in Tournament Mode.
Well, that's all I know for now.