Their house, their rules. So basically, the rules of their house you have to follow but what goes on outside of the house is your life and your business, not theirs. You're 18 now. Where I live, one can get married, vote, drink and be legally recognized as an adult with their own rights at age 18.
you tell your parents you will follow the rules pertaining to their home but they have no right to govern your life for you. That's like telling you that because you live in their house, you have to vote for their political party, you have to drink only what they drink, etc etc. They don't have such rights and that includes making you go to speech therapy.
You're in school, you're in a band, overall, you're doing more than a-ok, you're old enough to know what's best for you as far as deaf matters are concerned, and it's time for you to claim that especially now that you are an adult.
