An airbox is required, (as opposed to the plain K&N style filter and nothing else). A 14-inch silencer should be enough to pass sound. That was my setup; R2 pipe with a 14″ RLV silencer and the 4-hole style airbox, and it passed. While the silencer and airbox matter, I don’t think the pipe itself, R2 or R4, makes much difference in terms of loudness.
And they will test for sound. Last time I raced there in 2013, they sectioned off a special area where they did a sound check. You had to pass the check and get the kart tagged (which was good for the season), before they even let you out on the track.
It is strictly enforced, set by the town the track is located in, and is beyond the control of F1 Outdoors to make any changes to it.