An integrating apparatus to lower the levels of audio outputs of an on-board audio system immediately upon cessation of a noise level such as when a vehicle comes to a stop. Audio signals from an audio source are amplified through attenuating means by an amplifying means to drive a loudspeaker. An output of a microphone for detecting a noise is inputted to each of two integrating circuits, which have different fall time constants. Outputs of the integrating circuits are applied to a selectively outputting means, which selectively derives an output having a lower level among the outputs of the integrating circuits, and which then supplies the output as a control signal to the attenuating means. When the level of noise is high such as when the vehicle is running, the control signal has a high level and the attenuating means reduces an attenuation. Thus, the output of the audio source may be heard without a drift of the audio signal level. When the vehicle stops, the control signal level is lowered immediately, whereby the attenuation of the attenuating means is increased which results in the convergence rate of a fall of the audio output increasing.