Well, since the air inside his suit is not moving around him, then he can hear sound through it. Had he not worn a mask while going above the speed of sound, he would be able to hear things in front of him, but not behind him. The sound behind him simply wouldn't be able to catch up to him.
sound travels at 343m/s per second. i assume he was going faster than that, so i think he could have heard thigns out side the suit, but have heard it much delayed i believe.