Maybe they don't like your taste in music. TV noise can be annoying, but music can evoke strong emotions. Or maybe you're repeating the same songs a lot. That can be torture to someone who doesn't like the song, or even bad for someone who doesn't mind the song until the twentieth time they have to listen to it.

You may be playing something that is not to your Neigbours ‘tastes’. People are so easily offended by what isn’t the ‘norm’. I think you should just try to play it only as loud as you need to hear it but shouldn’t be able to hear it in the hallway to avoid complaints. Maybe move the speakers away from the wall into the middle of the room?

Do you have the bass turned up? That’s the only time I’ve had issues with music as it reverberates and is impossible to escape.

It doesnt matter what i play i play anything except country and speaker is never against adjoining walls and we have that foam stuff up to help absorb vibrations you also cant hear it in the hallway already but they still complain we played music one night so low it was basically our phone volume n there was a complaint about hearing it on the first floor

There must be something different on how music sound travels. When we were first married we lived in an appartment with concrete between the floors. Shortly before we moved to our first house an East Indian couple with a little boy moved in above us. I never heard any noise of any kind except for their music. It drove me up the wall. To me it just sounded like different random twangs.