An audio playback apparatus, an automatic player musical instrument and a voltage controller form an ensemble system for producing music tunes in ensemble between the audio playback apparatus and the automatic player musical instrument, and the voltage controller is responsive to a volume control dial so as simultaneously to vary the loudness of electric tones radiated from the audio playback apparatus and acoustic tones produced through the automatic player musical instrument; when a user rotates the voltage control dial, an audio signal and a quasi audio signal are amplified, the audio signal is supplied to the sound system for converting the audio signal to the electric tones, and the quasi audio signal is demodulated to MIDI music data codes for changing the velocity from the original value to a new value for producing the acoustic tones or electronic tones through automatic playing or composition of waveform data.