WHY IS THAT?
It has to do with percussive overtones when the reed slaps against the mouthpiece body every time it vibrates. Two materials are involved, that of the reed and that of the mouthpiece. Different material combinations create different percussive overtones, which flavor the base tone of the pulsing air. That's why different mouthpieces used on the same body create different sound qualities when playing the same note, vibrating at the same rate.
Plastic reed against plastic mouthpiece is very tinny. Silicone rubber coating the plastic reed material helps; silicone against plastic. Silicone reed to bamboo mouthpiece is better. Silicone reed to silicone mouthpiece is best.
I have plastic drumsticks with rubber heads. Hit the sticks together and there is a sharp sound. Hit the rubber heads together and there is a soft dull thud.
Since there are fewer percussive overtones, the base frequency sound of the reed vibration rate is cleaner, and less tinny.
Be sure and hear the sound sample in the final step.
Step 1: Making the mouthpiece
The strip of X-ray film has an elongated hole in it through which a dowel passes. The X-ray film defines the curve and the dowel defines the air passage through the mouthpiece. Both get soaped with dish detergent and are allowed to dry. The detergent acts as a mold release agent.
I pinch the dowel to the film base with a clothespin. Between the dowel and the film base I insert a narrow strip of film material, which assures me the bottom of the air channel will be sealed if the silicone doesn't find its way completely under the dowel to join with the silicone on the other side.
As I build the silicone up and over the dowel, I add a strip of film material to the material over the air channel to help make the mouthpiece more rigid. This combination of materials gives me the silicone surface I want with more rigidity than the silicone alone would have.
I usually give the silicone at least half a day to harden up completely before removing it from the mold.
Next step, scrub the rubber with water and a tooth brush to get any detergent off of it. Let it dry.
Then, using the syringe, for precise placement of silicone, glue down a strip of reed material. You can test reed position by blowing the mouthpiece before the silicone sets up. When you get it the way you want it, set it aside and let it dry.
Trim excess silicone with scissors. Clean the inside of the CPVC fitting with some toilet paper and lacquer thinner, so silicone will stick to it. Stick the butt end of the mouthpiece into the section of CPVC pipe with a little silicone.
When the silicone dries, you are ready to play!
I made lots of mouthpieces. Each one comes out a little different. Experiment with different curvature. Collect your favorites.
Check here in my Pocket Tootophone instructable for tips on shaping the curve. http://www.instructables.com/id/edit/?instructableId=ECCXCO7H0468IA5#instructableId=ECCXCO7H0468IA5,stepId=SDXF3BNH1PIT8Z1 The diagram below, "Sanding the Curve" is from another instructable. There is no sanding for the silicone rubber mouthpiece, but the ideas behind proper curvature are the same.