A sound signal processor and a method for processing a sound signal using the same enables the elimination of a plop noise generated in the course of a sound mode checking operation. The sound signal processor receives a sound intermediate frequency (SIF) signal and includes a channel mode discriminator for determining a current sound mode of the received SIF signal; a mode transition checker for performing a checking operation to determine whether the current sound mode has resulted from a transition from a first sound mode to a second sound mode; an amplitude measurement circuit for measuring an amplitude of the received SIF signal; and a controller for controlling a drive of the mode transition checker according to the measured amplitude. If the sound channel mode of a currently input sound signal is a mono channel mode, the amplitude of the input sound signal is measured, and the measured amplitude is compared to a reference level. A process for checking whether the sound mode makes a transition from the mono channel mode to a stereo/dual channel mode is performed according to the comparison, whereby the process is either repeatedly executed or is discontinued. The checking process is prevented from being unnecessarily executed and thereby prevents the generation of a plop noise.