Please tell me there's a primary muffler and resonator further up the system that I can't see in the pics, or I will be forced to post a facepalm picture in your thread.
You'll probably find the cannon and dumpy tip are to blame... Dumpies stop exhaust noise travelling as far (outside the car) but have the exact opposite effect inside the cabin.
Also, straight through cannons (like that one, which looks like one of the $90 Trademe ones) suck, do nothing, and on small engines act as self contained echo chambers.