Horses, being herbivores, haven't developed talk as a way of communication. Loud noises would only attract predators. The basis of horse communication is the body language, especially head and ears position. Horses ears tilted back along the head are a sign of threat. I've read somewhere that horses obey to people because they believe we are constantly angry because of our ears. Which would mean that horses won't respect lop-eared people. :) There are stupid stories about horses as many as you want. :)
Horses easily read their rider's body language. If a horse senses your fear, it will become unsecure and wish to get out of the scary situation. If you are afraid, you need to learn to behave like you're not and everything will ok. Fake it till you make it.
Horses rarely use vocalisations. Maybe that's the reason why human voice has such an impact on them. If you wish to punish a horse, it's a good idea to accompany the punishment with yelling. They have a hard time listening to it and will do anything to stop it, or to start talking or singing to them again. A calm human voice soothes the horses. Once, two stallions met head to head and I was standing in between holding them by halters and yelling at them which stopped them from starting a fight right away. They started after 30 seconds. Too bad nobody was there to grab one by the reins. I wouldn't have my distinctive smile today. :)