Mike Hartman has been sharing his muffler modification for sound (and performance?) on the web for years. It's cheap and relatively easy with the right resources at hand.
While I have not
done the Hartman modification myself, I created this page for those that wish to and need the info.
Let me know if the info is correct and if it worked for you or not. Also, I'm not an expert on this
mod, so I can't help you much if you have questions.