From:
http://www.youtube.com/watch?v=Ie_R1H9L4eI
"As far as i know, Yoko only worked on the CPSI version of the game [Hyper Street Fighter II], it was mentioned that she did all but 3 songs in the original SF2. She had no involvement in SSFII, she was only credited because she was responsible for the tracks alone, same with isao abe, but if i am not mistaken, Syu Nishigaki was responsible for the SSFII Arrangements as well as the new themes that were non-existant in the CPS1 games."
-CosmoBuggi
*Note: CPS2 Hyper Street Fighter II = Super Street Fighter II = Super Street Fighter II Turbo
I'd say that since this is just an enhanced version of the original Street Fighter II, it would only make perfect sense that Isao would arrange the three he worked on (Sagat Stage, Here Comes a New Challenger, and Round Start/VS) and that Syun essentially replaced Yoko. As for all the new compositions (Fei Long having been confirmed), I think Syun did them all as well; they all have the same sound as the majority of the character themes.