NJM2072D Application Questions for Audio Detection Circuit
Judging from the block diagram, this is a DC blocking cap (/ AC coupling capacitor, however you choose to look at it). It looks like the input impedance is nominally 20kOhm. You would choose this value to pass frequencies above a certain cutoff. For instance, for audio, maybe you choose to pass above ~100 Hz: going through the RC filter calculations, you would need a 80 nF cap - so a standard value 100 nF cap would probably suffice.
The block diagram suggests that the 20k and 100k set the inverting gain of the amp stage. A resistor from pin 2 to pin 3 could be used to decrease the gain of this stage. For instance - right now, the 20k and 100k suggest a gain of -5. By adding another 100k from pin 2 to pin 3, the gain would be halved to -2.5.
The capacitor to ground on pin 5 is used to set the dead time from when the channel input goes quiet to when the outputs reflect this change. When the peak input drops below a set level, the internal switch to ground opens up and the 2uA current source starts slowly pulling up the voltage on the pin 5 cap. With a 10 uF cap, this is nominally about 2 seconds. (But with significant variation, as the charge current varies from 1 uA to 3 uA). With a 1 uF cap, this would be reduced to about 0.2 seconds.
The input to the NJM2072 is rated to a absolute max voltage of Vsupply - 1V.  Whether this is a problem depends on the supply voltage used for the NJM2072 and the peak voltage of the speaker output levels. If it ends up being a problem, you could attenuate the signal before coupling it into the NJM part.