A useful little application I sometimes use here in my studio is called The Levelator. You can drag and drop a WAV or AIFF file into it and it will "equal out" the overall volume of a file like magic. This is especially useful if you were to have multiple voices at different volume levels.
I sometimes will use this when I'm speaking very softly at the microphone but want the volume of my recording to be elevated. The best way to see what it can do for you is to try it out. You can download it here. There's also a more detailed explanation as to what it does.