Muffler

What is the muffler between the converter and rears mufflers for? If I put a pipe in there instead, will it be just a bit louder? Will that cause any other problems?

I believe that is the resonator. It muffles the sound even more. If you put a straight pipe in its plase it will be louder and give you a probability of making poping and/or raspy noises. It also creates backpressure that these cars need. I know of people that have taken off the resonator and they seem very pleased.