Not very often do I hear what I would actually characterize as a new sound. 20th century atonal, electronic bleeps and blurps, I've heard (and enjoyed) something about each of these. To actually sound new, I think there needs to be enough familiar about it, to get your brain to categorize it with other music rather than just another noise, but with enough changed to make it sound really different, somehow. Here's a clip which, to me, sounds different from anything I've heard before: microtonal blues.
Added: turns out this is from an album released in 1968, before I was born. New to me, anyhow.