For me, the classic SDC sound will always be the hammered dulcimer. In the '70s/'80s, it was a rare and intriguing instrument that few people had ever seen or even heard of. At the Main Street Gazebo, there was almost always an act featuring hammered dulcimer, and on some occasions you'd even find them at other spots in the city. Back then, it was common to see live music acts set up under shade trees throughout the park.
The funny thing is that the hammered dulcimer is such an iconic bluegrass instrument, but has totally different origins. Bluegrass and all the music of Appalachia is Celtic in origin, coming from the Scots-Irish settlers (Ulster Scots, or whatever the PC term is today). The hammered dulcimer came from Southwest Asia, probably Persia.
Another classic sound is the train, of course. And the roar of the furnaces in the glass factory. There's far too little clang of the anvil, since the "blacksmith" shop has traditionally focused on casting pot metal trinkets.
Ah, the clop of horse hooves... a long gone sound, but one that is fond in my memories. So is the loud bang (and rolling clouds of smoke) of real black powder blanks in Main Street shootouts and the train robbery, before they went to primer squibs.
Okay, the sound and scent memories have me calculating how to board 7 dogs for a quick weekend trip... somebody stop me!