http://www.epa.gov/region02/air/sip/13_20-Sub32.pdf
the only mention of a replacement muffler is on page 24, which says it's acceptable if the muffler is made for the vehicle and the company certifies that is has a safe exhaust gas exit.
i have yet to find any mention of sound levels, even in the motor vehicle code (which the cops use to write tickets). i'm tired, i'll look more tomorrow.