Ball gags come in multiple sizes, if the one you have is too big, they probably come in a smaller size.
Otherwise, why is your slave so loud? Does she not modulate her voice well? That can be fixed, find a consequence that she will respond to and impose it everytime she is too loud. Does she yell out too loudly during playtime? Cut playtime short everytime until she learns.
formally collared 1/30/09
"I give to you my everything, you've given me these loving wings." - DMB