Has anyone posted the inside of an OEM muffler? I just went OEM and when I was installing them I noticed the muffler was full of "junk". I might be opening mine soon, but I'm curious to see if anyone else has.

 

I have a thread where I cut these open if you want to look.

I almost put straight pipe, or a single glass pack in them. This is ideal because the weight of the muffler case (around 14 pounds) dampens the end of the tailpipe, and still wouldn't have much drone, but with maximum sound.
This is how a lot of high end manufactures like Aston Martin get away with the loud sound but no drone.
The stock mufflers sound good up close, but when the car drives away from you, you can barely hear the exhaust as it moves away.

 

I can imagine after 15 years the holes (preforations) inside the muffler are probably smaller due to rust buildup.

I did this on a friends 2007 Civic a while back. He wanted a just a tad of an exhaust note without actually changing anything. I drilled a few holes through the chamber pipes and re-welded the casing. There were no "gains", but he was happier with the overall tone.

I did see the inside of an IS300 muffler, which had an interesting sound flap inside.

 

A friend of mine cut open his ES300 muffler and it was very strange. It had a double wall in the middle of the muffler to section off three segments, two large air pockets flanking a very small (talking like half and inch to an inch here) inner chamber. The were three pipes.

An inlet pipe would pass through the first wall and dump into the small middle chamber. The middle chamber had a pipe that exited on either side of the double wall and into either larger chamber (intake side and exhaust side). The pipe that dumped into the intake side large chamber eventually exited through a pipe that passed through the double wall, out the exhaust side, and out to the exhaust tip. The pipe that dumped into the exhaust side chamber had a spring-loaded door that would open or close to fill said chamber if exhaust flow was high enough, which would actually bypass the whole intake side chamber by going straight into the pipe that exits the muffler through perforations in said pipe.

TL;DR
'97 ES300's have a flap that opens when you're under high engine load to increase exhaust flow and enhance engine sound through a complicated muffler layout.