An audio controlled switch where the audio is muted in the right way.

ON and OFF switching system for active boxes
Speaker boxes come in all shapes and sizes. The 'better' speakers are usually equipped with several loudspeakers, each of which is then fed a part of the sound spectrum via a filter.
Usually these filters consist of capacitors and coils with which the frequencies are separated and, if necessary, resistor networks to adjust levels. Such systems are difficult to design and only suitable for the combination for which they were designed.
It is also possible to use an active crossover filter and to give each loudspeaker its own amplifier.
Such a system can easily be built into a loudspeaker box, the active box.
There are currently many amplifier blocks for sale.
The design presented here is intended to take advantage of class-D amplifiers that are widely available. The intended types are based on TexasInstruments designs and are of good quality. These amplifiers can be powered with a single DC voltage that can vary from approximately 12 to 30 Volts.
Because nowadays many laptop power supplies are available from discarded laptops, it is attractive to use this. Many of them can supply about 19V at 3A, enough for a decent amp.
A difficult point with active boxes is switching them on and off. It is annoying if you first have to walk past the speakers to switch them on before you can enjoy the music. That must be different from the idea. 
In the next circuit, the box is switched on as soon as there is an audio signal. To this end, the audio signal is amplified and as soon as there is a little level, a comparator switches over. This controls a timer that immediately switches on the power supply and switches the audio signal through with a short time delay. When switching off, this happens in reverse order: first the audio is blocked and then the power is switched off. This prevents annoying noises from being heard during switching on and off.
However, it only switches off after the audio signal has been absent for an extended period of time so that pauses between tracks on a CD/LP do not result in a switch-off.
The circuit consists of 2 parts, namely:
1- audio detection system
and 2-time delay. 

The audio detection system:
This amplifies an audio signal of several millivolts, rectifies it and uses it as a switching signal via a comperator. The gain is limited using zener diodes. The 1N5225B was used for this in the simulation. The switching happened between 2 and 3 mV input signal. If other diodes are used, for example the BZX84A3V6, this can easily be 30-31mV. The choice of the zener is therefore a determining factor for the behaviour.
The comparator is provided with a positive coupling so that some hysteresis is created. This makes switching more stable.
In the simulation, the comperator is directly followed by mosfet switches so that we can test the principle. In the realization the comparator is followed by the time delay which in turn controls the mosfet switches.

The time delay:
A uController was chosen for this because it allows the required behavior to be obtained with few additional components. The AtTiny85 is extremely suitable for this. The uController is controlled by the comparator and in turn controls the fet switches.
After the audio signal is detected, the power is immediately turned on and shortly thereafter the audio signal is released. When switching off, this is done in reverse order. It will not turn off until no audio signal has been detected for approximately 5 minutes.
The schedule:

ImagePicture 2: total schematic, without decoupling and smoothing capacitors
The complete scheme is shown in Figure 2.
Led D1 indicates that the supply voltage is present.
Led D6 indicates that the power is switched through.
T3 is the fet switch that releases the audio, that means the fet is conductive when there is no input signal, PB0 of the AtTiny is then high, led D5 is off.

A single-sided PCB has been designed for this circuit with modest dimensions: 35x77mm. This is provided with the necessary decoupling capacitors and smoothing capacitors.

The power fet with which the power is switched can possibly be replaced by an ordinary relay.
Both TO220 and TO3P versions can be used for the powerfet.