: Debatable.

: Audible map ambiance is basically nonexistent in Halo maps these days (get
: off my lawn, whippersnappers).

: Why? Discuss amongst yaselves.

Yeah, there's less of it. I heard that the 360 actually is less capable on the audio side of things than the original Xbox; that might have to do with it.
I think it's a design choice but I also think the environments themselves are a big reason. We've mostly had less outdoor levels and less "regular" outdoor levels over time. In Halo 3 there are a lot of environments that are deliberately barren and/or alien enough that it's understandably hard to come up with believable ambiance. Halo 4 has this somewhat too. That doesn't preclude ambient sounds though. Maybe later designers, or the same ones, thinking about it later in the series, didn't like the Forerunner machinery noises of Halo 1 and 2.
There's also background-ish objects in the environment. I can only assume nobody at Bungie liked the Covenant communication crates making a modem-y sound once they heard it in the game*, and after that there were comm towers that just made a really low hum, I think? Come to think of it there were less plasma batteries (which hum) in Halo 3 than Halo 2, I think, and less in Reach, and then none in Halo 4, and the number of fusion coils also might have gone down. They might have also gotten rid of those little UNSC generators I remember shooting a lot in Halo 3.

* personally I liked it until 343 Guilty Spark, then I associated it with creepiness and stuff lurking around the corner and I didn't like it any more