The technique isn't soundless. When Kurenai used it, it seemed to have background noise so I wouldn't really say listening in is the best way to try and figure out somebody's position while being trapped in this illusion (unless you have enhanced hearing). However, as you said, the means of discovery is rather easy.
Yes, if you go to the Wiki and look at the technique (quite literally it's just the Hazy genjutsu), you'll see how the technique is.
This is incorrect, you won't see anything other than yourself (and potentially any clones you may have). Unless this technique was used in a game that suggested otherwise, the Bringer of Darkness turns everything into a pitch black void. This technique is purely ocular in nature. Though, again the fact that everything else with the exception of yourself is void in your vision, is a dead giveaway.
This one takes a bit more of a process since you can't just assume that appearing from the ground like they do indicates that they are illusions. You need to discover through trial and error whether or not these clones are real. Like you said they don't cause physical damage unless attacks are coordinated. However, you should also not that they don't disappear when hit. In the event you're not a dojutsu user or chakra sensor with powerful enough eyes to tell that there is no chakra network, you have to go on something else. When you attack them, they distort but they reshape as if made of some malleable substance. This is a property unlike any other clone. E-rank clones disperse upon contact, physical clones take physical damage and even the restructuring properties of some clones like rock clones, they still take physical damage that is noticeable.