Mixer and first and second engines are cascade-connected, and the second engine is connected to a speaker. In mode A, input signals to the first engine are output to the speaker via output channels of the second engine, and mixing operation of the first engine is performed via a console. Input signals to the second engine are output for monitoring via output channels of the first engine, and mixing control of the second engine is performed via a personal computer. In mode B, input signals to the second engine are output to the speaker via the output channels of the second engine, and mixing operation of the second engine is performed via the console. Input signals to the first engine are output for monitoring via the output channels of the first engine, and mixing control of the first engine is performed via the personal computer.