doggoner,
It is possible he came from an abusive home and figures he's going to get belted when he hears an angry voice. He may well just be of a timid nature and his previous owners got rid of him because they wanted a "big, bad Rottie with an attitude".
Either way, you need to treat him calmly, praise a lot, reward a lot, and use positive motivation for him to come to trust you. There are some excellent books out there that will help you train your dog in a positive manner. Some authors to look for are Karen Pryor, Jean Donaldson and Ian Dunbar. They all approach training from the dog's perspective and help you to learn to reinforce positive behaviors.
Anything else that comes up, no matter how silly or small it may seem, if it worries you, don't hesitate to post here. There are lots of really experienced people here who will give you good advice.
