I've noticed several times whenever I hear shooting (i.e. NCR shooting at some Legion soldiers off in the distance) in the game that I cannot tell what direction it's coming from. I turn left and right to see if either the left or right speaker (I'm wearing headphones) will be louder or for faint, depending on the direction I'm turning... but the gunshots sound the same in both speakers. In FO3, I didn't have this problem. If someone was shooting or growling or making any kind of noise, I could tell whether it was coming from the left or right.

Has anyone else noticed this?

I figured it out. Had nothing to do with the game. My laptop (Alienware M17X) has some audio software installed called SRS. Well, for some reason, the "premium" sound enhancement that SRS does won't work with my headphones. As soon as I disabled the premium feature, my headphones reverted back to playing stereo sound instead of mono. Weird.