First of all you should ask your parents to explain to your brother that although he does not follow halacha, the atmosphere that they want to retain in the house especially for the rest of their children is one that does follow halacha. Therefore its only right that if he needs to watch movies he should use headphones so that everyone else not be disturbed.”
This is actually very good advice. You have gotten a lot of good advice from many posters. You are not your brother. You alone are responsible for your actions. Your parents have the right to expect that in their house there should be a level of discretion coming from someone who resides there, to follow the house rules. At the very least,he should not blatantly flout those rules.
You need to keep the lines of communication open, to see why your brother has changed. He will always be your brother no matter what, and you can possibly be influential on him, if he feels you are not against him and love him unconditionally. This is not an easy situation, but neither is it an uncommon one, unfortunately. But I reiterate, though you have no control over your brother’s actions, you have every control over your own.