Because they are loud. I just dumped the audio into Audition and, as with Halo 4, it's like 343 is making us participants of the Loudness War as well as whatever battles we take part in in-game. On average the new sounds are twice as loud as the old (I saw the same thing in Halo 4 when comparing to Reach). Personally, this very rapidly made Halo 4 obnoxious to play with headphones because in comparison to literally every other game I own, it was constantly deafening. I shouldn't have to turn my volume down for one game when it's fine for the other 40.

The sounds are pretty good. Even when volume adjusted and brought into line with the original audio (though I'll admit a case can be made to give a few a boost, like the Sniper). Given the option, however, I'd likely eventually revert back to the old audio and leave it there.

But seriously. I'm annoyed that everything has, again, received a volume boost when one really wasn't needed.