Most "quiet singers" I have heard have a handicap that jinxes them from the start...they are in too low a key for their voice. A lot of people try to sing in the same pitch they use conversationally, and that pitch is designed for low volume with no interference or competition from other sounds, like guitars etc. For me, D is probably the best key, but of course the melody line you are singing has to be considered so you know you can reach all the higher notes. If you are sounding quiet, I would suggest you try to transpose one or two keys higher and see how you feel about it.