I'm going to deliberately speak softly to my mostly deaf grandfather-in-law
Do you always speak softly?
Yes, I do it all the time. It really pisses him off.
It might be your nature to speak softly but it is expected that you speak louder around the hard of hearing. They can't help their hearing but you can change your voice volume so it is only fair.
Nah. He will hear me clearly if he wants to, and then he can understand the things I'm saying.
Your grandfather needs your assistance to hear correctly and it is your job to help him with this. When you get older, you will want younger people to do this for you, so treat him with the respect that he deserves.