Thats an electric guitar with a single string being payed, and with the volume knob being rolled up, and down. They call it "swelling". Very cool sound.
I'm still baffled by how SFV's OST completely stomps over SF6's. This here is a good example, but Hillside Plaza and themes of characters like Cammy, Ed and G are too good. SF6 really dropped the ball hard regarding music.