do you have the remote? if so have you tried the volume on it?
Also, remove the face plate and bring it inside and use a hair dryer on the contacts. If it is moisture that can help.
Also, after your next mudding adventure, remove the face plate and make sure to wipe contacts with rubbing alcohol as it will help the moisture evaporate.
BTW, which kenwood unit? My buddy owns a shop and is a kenwood dealer