"Yes?" she asked.
she has no capital, because
"Yes?" is an element of the same sentence, which describes what is happening said element.
"Noooo!" He fell to his knees.
He is capitalized, because it's the start of a new sentence, describing what happened after the implicit "[whoever] said" of the quotation.
OK, but is the following correct?
"Why?" she heard him ask.
The reason I'm not 100% sure is that the part after the quote is not describing him asking, but instead describing her hearing him.
If that's wrong, then fixing it by capitalizing
she would be awkward, because then the separate sentence might imply that there's a second thing being asked. So then you'd have to go further to refer back to the quote as
this, but that's also awkward:
"Why?" She heard him ask this.
You would probably want add something to make it worth using
said in the first place, making it something like:
"Why?" She heard him ask this, but then everything went black.
"Why?" She heard him ask this. I could tell by the puzzled look it elicited from her.