I have a lot of free time. That means I can sit around and listen to stuff like Karlheinz Stockhausen's (1928–2007) Hymnen, a piece from 1966-67. This is in four channels. I listen to things through earphones, so I get a definite stereo effect. It is supposed to be made up of national anthems, some sung by choruses, with a lot of electronic manipulation.
The first comment says "The problem with all these 60's electronic piece is that they sound dated now as technology has progressed." Some of it sounds like needles on vinyl. That would definitely date it. There are 22 pages of comments.
I've never been in a situation where I could actually listen to this stuff.
Another landmark of the twentieth century is Yannis Xenakis' (1922-2001) Metastasis (1953-4) which created patterns from mathematical probability theory. He studied with Messiaen. I like the way this is displayed in YouTube, but it's pretty tedious. I generally think Messiaen is rather bizarre but interesting. Einstein is blamed.