The sound generator is just a plastic cylinder, with an airtight tube connecting it to the intake, and a sealed flexible diaphragm (like a speaker cone) on the other end. It's completely sealed and does not reduce air flow into the engine, as air does not pass through it. Intake air (vacuum) pulses cause the sealed diaphragm to vibrate, creating sound waves that you can hear on the other side of the diaphragm.
You can see the noise generator, elbow hose and clamps. You cannot add the sound generator to a Greddy or Injen pipe unless there was a way to plumb it up - which there isn't. If the sound generator could be plumbed in, it would add the noise that it produces to the overall sound. I'm not sure how the Greddy or Injen intakes sound on their own, so I don't know if it would offer a substantial difference or not.