The next time you play your favorite game, mute your speakers....notice how empty and dead the game is? play like that for about an hour then turn on the sound. Listen to every little sound the game has, from the player coughing to machine guns dropping brass shells. With audio the devil is in the details. Think about every sound something makes. Prioritize the major ones that would stand out if they were missing. Add a few extra ones that help the major ones. Then add little ones to give the game more detail. When adding sounds you are adding life to your world.