I have encounter countless customer angry over all sorts of things. At the peak of anger, they cannot be reasoned. The best way to bring senses back to them is to start apologizing, even if you are right.
Thats right, no matter how be the first to say "SORRY". By doing so, you are telling the customer that
1) you are for them
2) you are with them
3) you can help them
Then, when you calm them down, begin to reason from their point of view then lead them to yours. This way, many issues can be solved amicably.
Remember, only people who have no reasoning power will raise their voice to cover up. If they need to shut, let them shout all they want. Trust me, they will get tired too. This will be your chance to reverse any situation.
Hope my little idea helps.