One of my students had a similar problem when he started learning to play violin. He used to practice in his walk-in closet so that his neighbors and girlfriend wouldn't suffer too much. Actually, the problem was that the sound was harsh/screechy, rather than loud. The quickest, easiest thing you can do to alleviate the problem is to buy and use better strings. That will make your sound a lot more mellow. As you learn to use the bow better, you will also get a warmer, more pleasant sound. I suspect that's your problem, rather than loudness per se.