Ah...the sound question. One of the favorite FAQs
The jett-strem mufflers are slightly quieter than the stock muffler, running the same prop at the same rpm.
How much quieter? Hard to tell. Varies by installation, aircraft and prop choice.
However....
Since the purpose of the muffler is to increase power output and rpm, you end up with that same prop turning
higher rpm. Which will tend to increase the prop tip noise and exhaust note ... and the sound level ends up back to or in some cases slightly above the stock muffler.
One way to minimize any inpact, is by increasing prop size, to keep the same rpm you had in the first place (assuming the inital rpm was within the tuned range of the muffer to start with). Thus you utilize your power gain by turning more pitch or diameter. In other words, if you were turning a 12x6 to start with, you might try a 12x7 or 12x8.
Bob