Effects and Music just don't cut it in a game like this.
Gunfire.. (includes explosions, enemy fire)
Vehicles.. (Tanks are suppose to be loud but not THAT loud)
Ambient.. (birds, water, those buzzy cricket things)
Player Noises.. (footsteps, reloading, movement sounds, breathing)
Radio.. (Fuck.. 4 is down)
In Game Speak.. (this tree looks awesome over here.. What?)
Those are just a few off the top of my head. Having each one control the volume for your stuff and the enemy will make it fair. Wouldn't want you muting only your vehicles and making enemy vehicles so loud you can pinpoint their base like a bat.