I was checking the internal flappers on my through hulls yesterday, as soon water at my camp will be too low to dock transom in. I found the internal flappers were being held open by something forward of them. I think they are some kind of muffler. If you took a solid 4" plug, cut out a 2" center and moved it 1-2" out to allow exhaust to escape from between, that is what they look like. I pushed them back with a prybar to let the flappers move. I'm going to get riser gaskets today and remove them to see what happens. Anyone know what they are? I don't know what brand tips Sonic uses.

Those sound like the internal slip in mufflers that CP Performance sells. They are supposed to be mounted in the exhaust tip though and don't do much for noise reduction anyway.