"Low Volume" quilts makes sense if you think in terms of bright colors being referred to as "loud". Low Volume quilts are done in soft colors. Pastels, etc.
As for sources...when I was in Oregon I visited Fabric Depot and they had a HUGE selection of low-volume batiks. I don't know if you're interested in batiks for your quilt but if you are that might be a good source - they have an online shop.