a) sound depends on cubic capacity of engine b) design of cyllinder as well as piston for example as we know that there is cut  section at piston skirt in honda engine c) material of the engine d) exhaust system design also depend upon mist eliminator e) valve design f) engine type (horizontal or vertical) g) last but not least is its weight or design(aerodynamic)

Sound depends on engine tunning, how much of torque and speed is obtained varies sound over it the exhaust nozzle also plays great role. As per my knowledge