If you still haven’t figured out how to unlock the hidden characters, here’s how.
Both characters are unlocked on the character select screen.
For Akuma, you need to select Bison, Sagat, Guile, Chun Li, Ryu, Ken. You’ll hear the jingle again and Akuma will be available on the bottom left.
Because they play quite differently to the rest of the cast, you’ll need to unlock them each time you enter the select screen. But as a convenience, if you restart a game after playing as one, that character will be unhidden and highlighted for you to jump right back in!