I was a little worried before the game came out, having gotten used to the Halo sounds (like many of you, I'm sure). But I was pleasantly surprised, too. My sound system has a fairly nice bass kick to it and all of the weapons sound, and consequently feel, much more powerful. The vehicles have improved as well, in my opinion. I know a lot of people aren't happy with the Warthog, but has it ever really sounded like a proper car? I've always thought that the engine sounds were a little... light. It sounds much more raw now, and I've yet to be annoyed by it.
Definitely a good move as far as sounds go. The music is another story, though, but that's not related to this thread.