A volume controller (2; 2A; 2B; 2C; 2D; 2E) for modifying music data codes expressing at least pitch and loudness of tones to be produced, comprising: an estimator (16, 22, 23) supplied with a volume control signal (v') simultaneously expressing said loudness of said tones to be produced on the basis of said music data codes and loudness of other tones to be produced from audio data codes, said audio data codes expressing discrete values on the waveform of an analog audio signal and determining a target value of said loudness of said tones on the basis of said volume control signal (v'); a code restorer (16, 21) supplied with a music signal (m(t)), and restoring said music signal (m(t)) to said music data codes; and a music data modifier connected to said estimator (16, 22, 23) and said code restorer (16, 21) and modifying bit strings of said music data codes expressing said loudness to other bit strings expressing said target value of loudness so as to produce voltage-regulated music data codes, characterized in that said estimator (16, 22, 23) includes a multiplier connected to a sound signal generator (la, 11, 12) outputting said music signal (m(t)) and a volume control signal generator (14, 15) outputting said volume control signal (v') and multiplying said music signal (m(t)) by said volume control signal (v') so as to produce an amplified music signal (v'·m(t)), and an envelope detector (22; 22A) connected to said multiplier so as to calculate a ratio of the magnitude of said amplified music signal (v'·m(t)) to said music signal (m(t)) on the basis of an envelope of said amplified music signal (v'·m(t)), thereby determining said target loudness on the basis of said ratio. an estimator (16, 22, 23) supplied with a volume control signal (v') simultaneously expressing said loudness of said tones to be produced on the basis of said music data codes and loudness of other tones to be produced from audio data codes, said audio data codes expressing discrete values on the waveform of an analog audio signal and determining a target value of said loudness of said tones on the basis of said volume control signal (v'); a code restorer (16, 21) supplied with a music signal (m(t)), and restoring said music signal (m(t)) to said music data codes; and a music data modifier connected to said estimator (16, 22, 23) and said code restorer (16, 21) and modifying bit strings of said music data codes expressing said loudness to other bit strings expressing said target value of loudness so as to produce voltage-regulated music data codes, characterized in that said estimator (16, 22, 23) includes a multiplier connected to a sound signal generator (la, 11, 12) outputting said music signal (m(t)) and a volume control signal generator (14, 15) outputting said volume control signal (v') and multiplying said music signal (m(t)) by said volume control signal (v') so as to produce an amplified music signal (v'·m(t)), and an envelope detector (22; 22A) connected to said multiplier so as to calculate a ratio of the magnitude of said amplified music signal (v'·m(t)) to said music signal (m(t)) on the basis of an envelope of said amplified music signal (v'·m(t)), thereby determining said target loudness on the basis of said ratio.