There are several aftermarket mufflers available, and they do make the bike louder -- at least one even has removable baffles to make it really loud. Do a search on the keyword "muffler" for a list of threads.
Be advised, though, that although many people do so, in some places modifying the exhaust system to make it louder is illegal. The State of Washington, for example.
If you want a loud bike, why not just buy a Harley or something?
A forum community dedicated to Suzuki Burgman motorcycle owners and enthusiasts. Come join the discussion about performance, modifications, classifieds, troubleshooting, maintenance, and more! Open to all 200, 400, and 650 models!