K.N.
I have ADHD girls and I have a loud girl -- fortunately (for me!), they're not loud *and* ADHD. That must be a tough combination!
Have you had her hearing checked? And does she have allergies? I find my loud girl to be MUCH worse when her allergies are acting up. It seems her head gets stuffy & she doesn't hear as well, consequently she gets even louder. A few days of Claritin or Zyrtec, combined with a lot of liquids and constant nose-blowing and it seems to help.
Might want to have her hearing checked. Won't help the ADHD, but it might at least keep things down to a dull roar!
Good luck!