If you need a horn to sound when the airspeed drops below a certain level or when the power is cut and the landing gear is still up and locked, an electronic circuitry solution is the lightest, most reliable and most cost-effective way to go.
Exhaust systems do more than just get air into and out of aircraft engines. Exhaust system design, in terms of exhaust pipe length, diameter and tuning, affect the efficiency and performance of the engine, for better or worse.