The smog test does not test the muffler. However I don't recommend you take
your Chevy in for a smog inspection with anything wrong with it, it may
cause the smog technician a problem with inserting or keeping the emissions
probe in place.
One other thing to consider is smog machines in California are calibrated
once every three days, taking in a sample of ambient oxygen levels. This
calibration disallows the testing of vehicles which emit oxygen levels
exceeding preset base limits. In other words, if any problem with your
muffler causes more oxygen to enter the smog machine then that emitted from
your Chevy's exhaust stream, the test results would be altered; the smog
machine may even not allow the inspection to begin or continue.