There is an error in the circuit diagram. The dc12v is short-circuited. This is caused by the wrong drawing of the sd line. When the audio signal is in the form of a current of 0~20mA, it can be converted into a 4~20mA signal and directly control the frequency converter, which is better than using a fixed multi-speed. Or use communication to control the frequency converter. Both options are better than your speed.