Audio Ducking Device

I need a small device to inject an audio signal (from a message announcer) into an existing music sound system in a building. This would be an "automatic sensing audio ducking device". When the message announcer outputs an audio signal, the ducking device will turn down (duck) the music playing in the building to a background level allowing the message to broadcast at a volume that is clear and understandable above the music. When the message audio signal ends, the ducking device will turn the music back up to its original level. The music level changes need to be ramped (smooth) and not just an abrupt level change. The circuit needs to have "pot" adjustments for "ducked music level", "trigger sensitivity adjustment" for message audio input, "duck hold time adjustment" to keep the music ducked through short gaps in the message audio. I would be interested in a circuit design only or a working prototype.