This is the first game I actually complained/have a problem with sound
I have a built in soundcard on my motherboard, I have the latest drivers for everything that could possibly apply to this. I have heard what Halo 2 for the xbox sounded like so I do have a comparison

First Sound problem, Example when Cortana/Miranda/anyone talks over the radio in single player they sound so much louder and crack alot. I know it a radio maybe some of it is in there for effect but it just too horrible and muffled. They sound fine elsewhere though

Sound Distance. Sometimes in single player I can actually hear the elites taunt me when I am no where near them. For example on the Cairo station
on a big elevator leading outside the door open and there are three elites wearing jetpacks. they are way far off and it sounds like they are whispering into my ear, in space. Some sounds in space are crap too. For example when shooting the battlerifle outside the station it is muffled, and sometimes it is not.

Some sound are not played at all. Most noticeable in multiplay. It feels like this games gets overloaded with sounds then stop playing some. For example I was playing on Ivory Tower when up the air vent on overshield side a guy started shooting me with smgs so I threw a grenade zoomed in with my battlerifle I heard the first three shots go off, he made a "grunting" noise then all the shots following were not heard I didn't even hear him make a dieing noise.