Lol what an interesting thread. I mean it really depends on the video. The more hardcore stuff is usually on lower volume. Headphones are loud anyway which is what I use 99.9% of the time so I kept it pretty low.
I use headphones most of the time although I am surprised that they can be quite loud. One day I was watching some rather intense scene and I took off my headphones to go take a leak and when I left the bathroom I was greeted by the moans of multiple women in the corridor. The split second of confusion was met with the realisation that the sound was coming from the headphones in my room.
When it comes to porn, Audio mixing is something I never really think about it up until a performers' moan is scraping my inner ear till it bled. would mind blasting the volume but those ear-needle moans is what's stopping me. so i have it at around 30% which is my comfort level for everything (Youtube, movies, etc..)
Probably about 40% for pro scenes and 30% for solo since half of that is shot on a phone and scratchy sounds can blow your ears out. Some of the best verbal videos have the worst audio. Truly cursed.