My son has SPD, like some of the other parents talked about. The fire drill story reminded me of this. Last week my husband was looking for the cordless phone and pushed the button on the base that makes the phone beep so you can find it. Our son was going to the bathroom and all of a sudden we heard this big bang and he was crying. It turned out the beeping of the phone sounded like the smoke detector to him, and startled him so much he fell down. : ) Poor little guy!
Usually his sensitive hearing causes him to cover his ears, though. Earphones really help and he isn't shy about wearing them. My dad has made comments a couple times - just a different generation, they don't understand (think everyone should "tough it out"). But other than that, it works for us.
The other funny thing my son does, is he likes to smell EVERYTHING. He seems to crave smells. Hates most noise and movement, but we went on a boat ride this weekend and the low drone of the boat motors totally calmed him.
Just keep an eye on your child to know what he likes and dislikes, and you will be able to help make things easier for him. My son does get sensory integration therapy from an occupational therapist, but you can do some of the same just by exposing him to the things that he's uncomfortable with, in a fun way in a controlled environment. With my son, even things he doesn't seem to enjoy at the time, later produce big results AND he talks about them like they were the best thing ever. Last year we rode a ferris wheel and K was only 3, cried and whined and wanted to go home. The next day he rode a noisy battery-operated tractor that he had been afraid of all summer, and for months pointed out where the ferris wheel had been and wanted to know when it was coming back. Just an example also of how stimulating a different sense can "overwhelm" their brain enough that the sensitive one (hearing) simply can't be as sensitive as it usually is.
Hang in there! Having a unique child is kind of fun sometimes.