I was playing Firearms (Half-Life mod for the uninformed), and almost ALL of their maps have really kick ass ambient noise. It's mostly random gunfire mixed with artillery explosions (incomming and outgoing), although there were maps that had screams and shouts added in as well. It SERIOUSLY added to the realism of the maps. You actually FELT like you were in a warzone. If anyone is planning on making a "warzone" map (like WarTornCity), I strongly suggest adding in ambient noises like that. Believe me, it makes a big difference.