IMHO graphics are as much important as sound , and 'style' of graphics + sound is what separates games. If you think about it , 2 futuristic games , Quake III & Unreal Tournament. Gameplay is very similar. Guns are like in any other futuristic action game. Whats the diffrence ? Style. From sounds & music , to level design & overall impression from textures & other visuals.

I wouldnt suggest you use dummy sounds in development , because it can give you a wrong impression of the game. When you change them , the game wont look the same... Better method is to choose carefuly the sounds for a game before you start making the levels & the monsters.(Atleast if not final , something very similar to what the game sounds will be) Listen to the music while you're working on the gameplay,storyline & level design , and you'll easely create the atmosphere you want your game to have.

BTW , grenades affect hearing even in a smaller game like Soldat. With realistic mode turned on , you never know if someone is behind you shooting at you , until you eighter hear him 2 sec. later or die so , sounds are important too.

PS.: For music , I sometimes use FruityLoops.