"Mufflers required" - well kinda, but usually it's not a matter of a muffler being required, but a max db limit specified by the track management. If the car's louder than the db limit (for instance, I think Road America is 103db), then you could have a problem. If it's not deafening in the car or you're ok to drive on the street without drawing undue attention or tickets, it may be ok for the track. If it's too loud, you may need the muffler in order to comply with the db limit (as Steven mentions, check w/ event organizers and/or the track if there's a limit and what it might be). If the track has an imposed limit, you wouldn't want to push your luck, would be a bit of a drag to get there and not be able to drive for the weekend, and no refunds either. The flip side is, if it's too loud, it's self-defeating - being a few-hundred rpms off on a shift point one way or another is going to be substantially less important than being able to hear, interact with, and learn from your instructor - and if you can't hear your instructor, you'll be struggling, making the whole event more difficult for the both of you, and perhaps more of a waste of your time and money than the fun learning experience it ideally should be. Also, you're going to be in the upper rpm band almost the whole time anyway, which should be much more audible than idling and toodling around town - assuming your street driving is typically not near-redline 3rd & 4th upshifts, or near 1g braking with heel & toe downshifts.