Was Flash able to hear gunshots or explosions at super speed or was he deaf to his surroundings?
Yes, sound is within the Flash's toolkit as long as his movement does not exceed the speed of sound. As soon as he does, only sounds directed at him, sounds he is heading directly at, or communication with other speedsters, remain within his ability to hear at superspeed.
The Flash is primarily a visual-stimuli hero. But he has been seen to use his hearing when he was working at subsonic speeds. In the early depictions of the Flash, sound was just as an important element as sight, helping him define his enemies, their positions and their activities.
To be perfectly honest, the short distances between the Flash and most of his opponents, he never need be to be moving faster than half the speed of sound (767 mph = 1126 fps / 2 = 550 fps) to get his work done. He can cover the distance of two football fields in a little over a second. The world would sound somewhat normal as long as he kept his speed below half the speed of sound.
As the writers ratcheted up the scale of problems the Flash had to deal with, he was forced to work faster and faster until supersonic speed was almost the norm. Once he started working at supersonic speed, sound simply wasn't an issue in many of the stories. So for the most part unless the Flash is communicating with another speedster, the Flash probably spends his time in a world that is mostly silent until he slows down to the speed of sound.