The sound adds a fourth dimension to the flightdeck. In the above picture you can see the two small amplifiers for the flight deck Channel 2 & channel 3. Inexpensive but adds a nice element.
This left & right sound channel is driven off of the P3D Server and only plays engine sounds.
My Instructor station drives sounds to the speakers above the captain and first officer. This left & right sound channel only plays call out sounds. Warnings, GPWS, etc.
My Captain Server drives sounds to the speakers install in the back wall above the circuit breakers. This left & right sound channel only play cabin calls. These cabin cables are included in the Sim-Avionics sound package and add a flight crew element to the sound experience. Very cool.