Musical sound signal is input to an electronic volume control unit, which in turn adjusts the gain of the input musical signal and outputs the resultant gain-adjusted signal to an amplifier. CPU supplies the electronic volume control unit with a sound volume setting level corresponding to an output of a rotary encoder. At the same time, the CPU generates, with reference to stored data of a ROM table, a hue control signal for changing a displaying hue of a three-color light-emitting diode in accordance with the sound volume setting level supplied by the CPU. The CPU takes in a current sound volume setting level when an upper-limit setting switch is activated, and it retains the taken-in level as an upper limit value of various sound volume setting levels that are to be used to change the displaying hue of the light-emitting diode between purple and red hues. As the sound volume setting level is changed from zero to the upper limit value, the displaying hue changes in a stepwise manner.