Is a stall warning horn required to fly?
No, many aircraft don't have them, like the Cherokee since it uses a gear warning horn. The horn signifies that the gear isn't down and you don't want to confuse that with the stall warning, so they don't have one.

23.207 says that an audible or visual indication is not required as long as the aircraft exhibits a "warning" 5 knots before the stall, in the case of the Piper Comanche the warning that satisfies the requirement is the stall buffet.

The relevant section of 23.207 is part (b):


  (b) The stall warning may be furnished either through the inherent aerodynamic qualities of the airplane or by a device that will give clearly distinguishable indications under expected conditions of flight. However, a visual stall warning device that requires the attention of the crew within the cockpit is not acceptable by itself.


So the stall buffet, as long as it is pronounced, is adequate indication of a stall to satisfy 23.207.