I believe ALL cars must use some forum of mufflers at the track.
The only time this is not a problem is vintage.
Open track day sound restriction is 75 dB.
I believe a regular race weekend is the same thing.
Vintage does have a sound restriction but it is so high most cars wont touch it (probably something in the 100s)
Probably a good idea is to check in with tech. Dave Lewis or Larry Jacobsen will be the person you want to talk to. You can send a email here firstname.lastname@example.org
Waterford Hills takes sound a little more serious since its through a township ordinance and court injunction. Though the sound is for the property line, once a car starts to get very close or bust sound i seen the people in sound (up on the wall) start to hang over the wall to get a good reading. I even heard that they will get a person to take a reading over at start if all else fails.
We do have a few spitfires that run the track regularly. Hoping at one point they cherp in.
In my opinion I say put on a small exhaust and dump it out the right side. Even if you dont run exhaust a straight pipe to the right will at least help somewhat.... till you get to the esses at least.