1. A system for estimating a torque value or a rotational speed value of a motor, the system comprising: a sound receiving device for receiving a sound signal of the motor when the motor is in operation; a feature extraction device for extracting at least one sound feature value of the sound signal; and an artificial neural network module for receiving a plurality of sets of training data to build a motor model for the motor and inputting the at least one to sound feature value of the sound signal to the motor model to compute the torque value or the rotational value of the motor, wherein the sets of training data comprise a plurality of historical sound feature values of the motor and a plurality of historical torque values or historical rotational speed values of the motor corresponding to the historical sound feature values respectively.