What I mean is that the loudness of a crossbow affects stealthiness.. or does the audio level of the sound file actually make a difference? That would be interesting.
The sound file makes no difference to stealth. That's just a noise the player hears.
The first question is do weapons effect stealth? I have never seen that explicitly stated anywhere.
If they do, then the game-code will assume that X-amount of noise is made by firing weapon.
INTERESTINGLY: Look on the Form for your crossbow. Find the "Art and Sound" Tab. At the bottom right of that tab is a field named "Detection Sound Level". Amend that value to be "Silent". Does that work?
(note that a LongBow is already marked as "silent")
If that does not work, maybe someone else has done this and can help you