Not “Sonic” as in the hedgehog variety, but sonic as in sound. Built by Dr. Cossack and the Harts, Sonic Man has the ability to receive and send out radio and sound waves of many frequencies, as well as the capability to use this to remotely reprogram other robots. He later installs a Needle Shooter weapon into his own chest. The special pellets this cannon fires are able to penetrate robots and render them inoperable for a short time.
Note: Yes, I’m going to be in trouble if Capcom ever makes a Robot Master named “Sonic Man.” Just remember: theirs is official, and mine is not!