Jane, that's a great solution, but feeds into my typically passive-aggressive nature, which I'm trying to improve on. I could see myself stuffing my ears with the headphones and BIL would say, "Oh, you got headphones?" and I'd have to say "Yeah, your music is kind of loud, so I got the headphones to mitigate it. Don't worry though! Play the music as loud as you want!"
That's the kind of behavior (in myself) I want to avoid. So, instead, I'll just unload on you guys.