I can't answer this one, for obvious reasons, but I think it's a fascinating question. I may have to ask it of my own hearing friends (with due credit, of course).

The sound of a 14.4 baud modem connecting?

Easy: The sound of the screen door's spring opening and then the frame slamming shut at the cabin my family has rented for 30 years in Conover, WI.

The eerie, hair-raising shriek of a bobcat in our backyard a couple of decades ago.

The sound of a helicopter flying overhead, the last vestige of combat in Vietnam.

Honestly, I'm not sure I wouldn't forget just about any sound eventually. Sort of like smells. You forget them until you smell them again, and they conjure up all sorts of old memories. Though the sound velvet makes when someone touches it makes me break out into a cold sweat just thinking about it. It actually makes me physically ill.

Sounds I hope I don't forget: Happy dog snuffles in my neck. Happy lover sighs. The sound when you sing along at a concert at the top of your voice and you can't hear it because everyone else is singing along too. + many more...

Sound I wouldn't mind forgetting: My sister screaming when our mother died.

I concur--a fascinating question! I hadn't thought of the modem, but definitely one sound I won't forget, perhaps because of the anticipation that accompanied it.(In that same category I'd put the sound of our old telephone being dialed...particular the sound of the dial returning to its original position.) The sound of a kitten screaming his first breaths (we named him Pavarotti); the opening strains of certain tv shows, like All in the Family; certain individuals' unique laughs, like my college advisor, or aerobil...

My mother singing to me.

My family's first dog, Sooty, made goofy strings of happy vocalizations that we eventually dubbed "runting." I can't remember why we called them that, but I'll never forget those sounds. Little guttural tones that varied in pitch and length, and signified pure doggy happiness.

I was the foreman for a murder trial. We found the defendant not guilty of premeditated murder, but guilty of reckless homicide (under three strikes, it was a life term). When I read the not guilty part, the victim's mother let out a shriek of anguish that cut a groove into my heart that I'll never lose. She thought I had let her son's killer go free.

The creak of an ironing board as my mother ironed. The slam and rattle of the screen door of my granny's cabin. My father playing Wildwood Flower on his big Gibson guitar.

When I was a kid and would sleep in extra late on Saturdays, through the grogginess of sleep I would hear my mother singing along with the stereo while she cleaned the house. Such a pleasant way to wake up (as opposed to during the week when our mother would invade our bedrooms and sing her "morning medley" while jumping up and down on our beds to get us up for school).

The sound of a black-capped chickadee in early spring. My dad whistling Bolero while cleaning the basement on Saturday morning. Marvin Gaye singing What's Goin On? Keith Jarrett playing The Köln Concert. My mother exclaiming "Roseate spoonbills!" Loki, our African gray parrot, whistling an incomplete phrase of the 1812 Overture.

The sound of a horse moving (walking, trotting, cantering, galloping and especially the sound they make jumping, right before they leave the ground.) and at the same time feeling the movement.

I've been writing about movies lately and I feel almost sick with nostalgia for the mechanical sounds that used to accompany film-showing: the hum of the motor, the whirl of film, the flap when it came off the spool at the end...