I know exactly what you mean!!! I was so frustrated having them only respond to my screaming. This is how we have handled it.
The 1st key is to come up with a CALM VOICE trigger that they know you mean business and there will be consequences for not obeying this time. This takes the place of the yelling trigger. It can be anything. My parents used my middle name when talking to me. When this happened, I knew Mom was not joking around. My particular trigger is to ask them "Are you ignoring me?" and then state calmly that "Because the consequence for ignoring me is ......."
The 2nd key is to make sure there are consequences EVERY SINGLE TIME they disobey after giving the trigger. The consequences have to be BAD and immediate. Make sure they understand the consequences and then follow through every single time. Don't ever use the trigger if you are not in a position to follow through. Believe me, they will test you!!! I had to jump out of the chair while my hair was being cut, chase my children and then punish them in front of everyone. It was incredibly embarrassing for me and worse for them, but now they don't act up when I'm getting my hair cut anymore.
For me the consequences are constantly changing, because what they care about is constantly changing. They got to where they were never watching TV because they kept getting it taken away. Now they never watch it even when they can. Same for video games. So neither works as a consequence anymore. As Dr. Phil says - "You have to find their currency" then don't back down! If you say without thinking to "Stop, or I'll cancel your Birthday Party", you better be prepared to call everyone you invited and cancel it. Because they will call your bluff!!!
I can also say, it is never ending. They are constantly testing. Prayer is the best help for us to keep us from getting beaten down and choose the right punishment, and for them to help them see the wisdom and have the grace to do the right thing.
Remember Delayed Obedience is Disobedience!!!
Good Luck.