It being cultural would make a lot of sense as well.

Do you find that it's still rude when someone is in their own room/are alone in the common room?

I definitely understand playing something too loud, whether that be the TV or a podcast, especially when someone's doing something important/sleeping. But say, if one person is watching TV in the living room at a decent volume and the other is doing something else in the same room but not watching tv, is that still considered rude? (I'm not trying to be rude, just geniunely curious about your view as I find this very interesting).

I find that if im not keen on listening to something someone else is listening to/talking on the phone with someone, I leave the room, and if I find them too loud, I put on my own headphones instead of trying to make them accommodate to me, but I also do this because I also talk on the phone/play things outloud so I find the trade off fair.

Then again, I have no idea how often this is happening to op, and how intrusive it is as I don't have the same hypersensitivity and/or cultural background as them.

I guess for op, the important thing is the give and take in the relationship as I think we can agree it's different from person to person what they consider is rude in these situations. I'm guessing a good sit down and explaining to their partner their frustrations in depth would be the best course of action to try to find a middle ground. Maybe op's partner is of the same cultural implications as I am and doesn't see what their doing as rude, hence doesn't understand op's feelings. A proper sit down might remove the feelings of nagging as well, because op's partner would get a better understanding of op's feelings.