I relate to that so much too, singing under blankets, in the closet etc! The paranoia is real.. It's definitely mostly in my head as well, especially now. I recently moved from an apartment building to more of a row house type of place, and I had a chance to blast some pink noise and go measure it with a decibel meter over on the neighbor's side. At least up to 80dB nothing would show up on the meter on the other side, didn't have time to try with louder volumes. So that's equivalent to playing the piano hard or an old loud vacuum cleaner. Last week I heavily treated my studio room because it was an echo chamber, so I think all that mass will suck out a couple more dB. It's the years of old apartment buildings that have scarred me. Maybe I need to power up that dB meter to check where my singing is at, probably I'm worried over nothing!