I heard something about drilling holes in the back of the muffler to improve sound. Has anybody ever tried doing this? I'm looking for a cheap way to improve the sound and thought I might give it a shot.

I did this almost a year ago. Others have done some form of it since. I never heard anyone badmouth it. I like it myself. This pic shows 8 holes equally divided on a 1 5/8 diam circle. The holes are 13/64 The exhaust is somewhat louder, and sounds good to me. You may want a little bigger holes, but I think 1/4" would get a lot louder.