Well, it depends. If you are going to be in a huge battle, and it is always going to be ravaging on no matter what the player does, then bullets and screams make sense. If the player''s actions can result in the battle going quiet, then that kind of ambience would make no sense as the display would show a bunch of people standing around.

As far as how you would go about making the sounds, be creative!
Get a microphone and record all sorts of sounds.
Find some kind of free mixer if you can, and then mess with the sounds.
I read that someone took a .wav file of himself breathing and he was able to manipulate it enough to make it sound like an explosion in a Space Invaders clone. You can surely find a similar way to make the ambient sound.

The thing about ambient sound is that it''s not very clear - especially in the case of war ambience. If you play the ambient sounds in medal of honour, they are low pitch, bassy sounds. The higher frequencies have been lost and it''s more of a ''thud'' sound than a ''bang'' sound. Take your bangs and booms, lose the screams, unless you want up-close ambience, and drop them in pitch, increase the bass and see what happens.

Maybe order them more logically too. Mortal sounds are usually preceeded by the whistle of the shell. Machine gun fire would be done in short controlled bursts as ammo was scarce during the war.