An audio system is constructed by a personal computer and an audio device, which are connected together by way of a serial bus such as a universal serial bus (USB), for example. The audio device such as an audio component stereo system provides at least one audio source such as the tuner and recording media, which provides first audio data representing music or else. The personal computer uses graphical user interface (GUI) for controlling operation of the audio data, wherein it provides control data and second audio data representing message sounds, which are transmitted to the audio device via the serial bus. Thus, the audio device performs mixing between the first audio data and second audio data on the basis of the control data. Then, speakers produce sounds based on mixing result. The audio device is equipped with an analog mixing circuit, which performs analog mixing between analog signals corresponding to the first and second audio data respectively. Or, the audio device is equipped with a digital mixing circuit, which performs digital mixing between the first and second audio data. Thus, the user is capable of hearing the message sounds while listening to the music provided from the audio source of the audio device, for example.