Moving silently through any terrain is a skill which has saved many
a life. The difficulty of such an attempt is based on factors such
as what is underfoot, and how much ambient noise there is.
Stealth checks are made by the GM. Except in extreme cases (usually critical success or failure), characters will always think that they are successful, and may be very surprised to discover the opposite.
If the Stealth check is successful, other characters' chances of hearing will be decreased by the amount the check succeeded by. If the check is failed, the character will be more likely to be heard, by the amount the check was failed by.