The horse she is riding is not even hers; she is training it for a client and has had it only 20 days at the time of the ride shown here. Note that she rides without a saddle or halter and bit and (obviously) without voice commands. She is using the method used by North American Indians. When you think your voice can't be heard I want each of you to watch this Video. She's not using her voice.... Yet she has persevered and achieved!
BTW, this has been on FB recently and Stacy Westfall is not a deaf mute. She uses 'no voice' to train her horses and no equipment which in itself is inspiring.