Q. Can a person who needs a hearing aid, adjust the volume on Shabbos? With a shinui?

A. Igrois Moishe (O.H. 4: 85 – see last question) permits the use of a hearing aid, since in reality the melocho is not done by the hard of hearing, but by the one who addresses him, and he is “eino mechaven” (does it without intention). Igrois Moishe maintains that it is not an unavoidable melocho (psik reisha), prohibited also when done without intention, since the hard of hearing may not even be listening.
Shemiras Shabbos Kechichoso (34: 28), quoting Minchas Shlomo, permits using a hearing devise when it was left on before Shabbos. He also permits adjusting the volume when necessary.
Horav Shlomo Miller’s Shlit’a opinion is that when possible it should be done with a shinui or using an unusual way for accomplishing the task, such as the back of the finger or the knuckles.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a