From pages 12-13 of the WI DNR Snowmobiling Laws booklet:
Your snowmobile must have a muffler in good working order, which
blends the exhaust noise into the overall engine noise. The only exception
to the sound laws are when you operate your snowmobile during a Sanctioned
Race or you are a dealer/manufacturer testing the snowmobile on
your own land.
Noise limit—for every snowmobile manufactured on or after July
2, 1975, the noise level standard for exhaust and engine noise shall
be 88 decibels as measured in accordance with the procedures established
for the measurement of exhaust sound levels of stationary
snowmobiles in the January 2004 Society of Automotive Engineers
Standards J2567.
You can’t refuse to allow your snowmobile to be inspected for suspected
sound violations or other suspected equipment violations.
You cannot operate, offer for sale or sell a snowmobile that is wider than
48 inches if it was manufactured after May 7, 1994.
You can’t operate a snowmobile off of the designated trail; if you do you
are trespassing.
Snowmobilers must stay on the right side of the trail and proceed
with caution when approaching oncoming snowmobiles.
Tampering with an odometer or hour meter is prohibited. Contact
DNR law enforcement at 608-266-2142 for directions on how to
replace or service a meter legally.
To help stay compliant, do not alter your exhaust to be louder than
the original exhaust system. If your snowmobile sounds louder than
normal, you may be in violation. Maintain your snowmobile and
seek the advice of a service technician if you are in doubt.
You cannot modify your snowmobile’s exhaust in any manner that will amplify
or increase the total noise emission above that emitted by the snowmobile
as originally manufactured, regardless of date of manufacture.
Excessive or unusually loud snowmobiles are illegal and hurt Wisconsin’s
trail system. Do not increase your exhaust noise or operate with
your exhaust system in excess of the sound limits. Wisconsin vigorously
enforces the sound limits and penalties can be severe, however
the worst thing about a loud snowmobile is that it causes properties
to be permanently closed to snowmobile riding.