[akomtu]

There is a lot of amateur literature on this topic, written by Monroe and others, even though Monroe in particular made an honest attempt to tell what he knew. To my knowledge, some dzogchen and especially bon authors talk about this, although very reluctantly. Among the western authors, the most direct work on this matter is Leadbeater's "astral plane", but if you're looking for instructions on how to obtain such clarity of perception, you won't find anything, except the usual "learn to control your mind and everything will follow". I think the reason is partly to fend off lazy curiosity, but mainly because many who already have all that's necessary don't need any secret instructions, and those who are not there yet can't use a safe method. It's quite possible that in the near future this will provoke a social strife between those who can and those who can't.

I'll make a guess that the loud sound is just switching audio source. Sometimes the new source is some internal process, like breathing, but unfiltered. I personally heard my own breathing in this transient state that was amplified to the loudness of the Niagara falls, and "colored" with the same waterfall sound texture. Even though at the mental level I recognized the pattern of my breathing, it was hard not to yield to fear, so overwhelming the sound was.

[spacebacon]

I think you have pointed out something very important here concerning social strife. Even outside of this topic there is already a huge gap in understanding between those who perceive and those who judge.