lose the muffler altogether. Straight pipe from the cat back. If you can get the o2 sensors to fit in the system without the cat and so it doesn't throw codes all day, do that and put pipe in place of the cat too...make it big pipe, like 3 inch or so. You could take the hood off to let more noise out, remove the carpet and any sound deadening from the floor and firewall. No air filter should help too, especially if you replace the factory plastic intake tube and put the flow sensors in a short straight tube right off the intake manifold. Advance the timing till it spits raw fuel into the exhaust and backfires/misfires...
These will make it louder...why anyone would want to is beyond me. The guy in the Supra/Mustang/GTI/SIR/WRX/ETC/ETC beside you at the track is not going to be intimidated by a yaris short of having a big blower sticking out of the hood...