That squelch noise is def a belt slipping. You can't mistake that... Although I have never heard a wet clutch slip real bad. Maybe someone that has can chime in on the noise it would make if any at all. Also, make sure your sheeve halves are clean and free of oils and not glazed over. I always take 300 grit to them, and clean with a degreaser before I place belt back on.
Backfiring is your jetting, or possibly a cracked header pipe close to the exhaust port of the head. Where are you located? Maybe someone can point you in the right direction of a good Rhino repair shop if you decide to go that route.