As a general rule, cars are required to have functioning mufflers installed just about anywhere, regardless of overall sound. erod550 is somewhat correct, too, in that many aftermarket mufflers could be considered illegal in many areas, but it's because many states still have vague sound laws that usually boil down to "it's illegal to make your car louder than stock". Some states, like CA, have actual db (decibel) limits one has to abide by- here, it's 95db, which is really pretty generous. 95db is loud enough for most, but still liveable to the general public. The SLP Loudmouths yell03 mentioned almost certainly wouldn't be 'street legal' here, but the Hushpwer axle-back is, along with others, I'm sure.