I'm not very knowledgeable about music, but here are few of my thoughts regarding Fallout music.
Taken into account the last few Gamebryo games, and in which way it has progressed, we can obviously divide music into two distinct parts:
1) radio music;
2) background/ambient music
About radio music...
First I'd have to say adding radio music was a bit of a dubious decision. Whereas having radio stations and listening via your Pip-Boy is not something I mind (I actually like it), there are a few problems about it, problems regarding the overall contradictory nature of the music that is played in the game, considering the world and the setting.
We've heard 'Maybe' and 'A Kiss to Build a Dream on' which are two great songs, and in my opinion, 'I Don't Want to Set the World on Fire' is probably the most fitting for the setting - but we've heard those songs in the games' intros. It is fine of course, but in the first two games we haven't ever been given a clear notion of what was the popular pre-War music. Few references here and there, but nothing big. We could only assume for ourselves.
Now, with Fallout 3, developers went in that way to add 30's - 60's music - the choice of the songs is great in my opinion, but they've missed the genre, and the point.
It seems that for over 100 years, no new music genres and styles appeared whatsoever. This was additionally supported when NV came out - same thing, yet again. Some different music styles, but it's the same time period.
I'm not saying that things like dubstep would evolve in Fallout's world music, but adding 50's songs was a bit illogical. As if the whole world went on, but the music evolution is stopped right in its tracks. Either that, or only 50's vinyls survived the War...
Ambient and background stuff...
I'm not going to write how awe-inspiring Mark Morgan's tunes were, or what an outrageous plagiarism they were, but we can all agree on one thing - music he made was well-fitting the setting, and defined the Fallout world in many aspects.
Again, in Fallout 3, developers went wrong with this one.
I'm not going to simply bash Fallout 3 for everything, as, in reality, it had many things faithful to earlier Fallout games, but music was not one of them. Not that it was bad.
On a contrary, I find Fallout 3 to have an amazing soundtrack - it was, in my humble knowledge, a well produced piece, with some quality tracks which had a nice, catchy tune to them. Problem was, they weren't really fitting to the setting.
I personally find
some of the things Inon Zur did to be very fitting for a post-apocalyptic setting - just not a Fallout one. Generally, yes, that soundtrack would probably go better with an epic fantasy game, but like I've said, I could easily imagine it in some other work of post-apocalyptic fiction.
All that taken into account, the little experiment Obsidian did with NV was certainly an interesting one. Whether it had 'best of the both worlds', or was an utter failure, it was a generally fitting soundtrack for the game. Some parts were annoying (Dam theme), but overall it had some appeal to it. My only criticism stems from the fact that some older Morgan's songs had been used poorly.
I can't really put a finger on it right now, but I think that eg. BoS theme from Fallout was badly used in NV, as in a wrong area. Though I may be talking nonsense right now, as I'm not really sure about the specifics.