Well there is a song on Fallout New Vegas that was released in 1997 so maybe music did evolve. At least in Obsidian's eyes.
Which one?
It's most likely just a 1997 recording of some 30s tune or so, or is a song done in that style. At least, that's where my bet is. Doesn't prove anything. Bethesda also did some recording for GNR, but those were jazzy tunes - again, fitting the perceived notion of retro-futuristic music which I personally find ludicrous.
Obsidian did make several new in-universe songs for the guitarist character, but those are country tunes, nothing radical. Pretty good ones, though, but aside from that, Obsidian hadn't showed any initiative in expanding or detailing the musical oeuvre of the wastes - not that I blame them for it - when you're making a post-apocalyptic RPG theorizing about how music could have evolved 200 years after almost all modern tunes have gone to dust is certainly not high on the priority list. They played it safe there.
Sadly, I think that all this discussion is without much point. As much as we speculate about it, it is a fact that music hasn't had a very proper treatment in the Fallout universe, as well as "outside", by the developers - a state of things which will hardly ever change.