So, let me list out some clarifications with his powers:

1. He controls the sound waves, not the source of the sound.
2. Volume amplification or muffling allows everyone to hear the sudden spike or decline in loudness, but he can only do this for sounds he locks on.
3. I haven't figured out a possible solution for sudden loud noises, although what Sep suggested -sending out muffling sound waves of his own to sort of lessen the effect it has on his ears- sounds plausible.