Analog circuit to check if an audio signal is present?
This would be easier if you were detecting signals at your -10 dBV maximum level.  But since you want this to detect levels as low as 10mV, you need to use comparitors.  You get 4 comparitors in a 14-pin package (LM339).

Couple of things to note: 

1) you need to ensure that the voltage differential that you want to measure is greater than the worst-case input offset voltage of your chosen comparitor.  A quick check with my old National Semiconductor datasheet says that the worst case offset voltage for the LM339 is 2mV.

2) these comparitors are open-collector.  They need pullup resistors to whatever supply voltage you need (+5V or whatever).  This is actually an advantage because it makes adding a peak-hold detector easy.

3) you need to bias the input to the comparitors HIGHER than your maximum expected negative peak signal and LESS than your maximum expected positive peak signal.  I usually set the bias to the power supply mid-point.

4) this going to be really expensive (grin).  Total BOM cost should be about a dollar for 4 channels.

I'll do a quicky schematic here.  Note that you will need to use 1% resistors in the input bias and comparitor reference section to get anything close to 10 mV sensitivity.







simulate this circuit &ndash; Schematic created using CircuitLab

Turns out that about the best sensitivity this will do is about 11 mV peak (22 mV P-P) and it will tolerate input signals up to about 2.5V peak.  You can improve the sensitivity by changing the bottom resistor on the reference voltage divider to the same 22.1K resistor used everywhere else and adding a low-value pot in series with that resistor.  Adjust the pot for your desired sensitivity.

Note that this detector includes peak hold.  The output goes LO every time the detected audio exceeds the reference voltage and decays back towards +5V when the audio goes away.  The time constant is currently close to 1 second but can be easily changed by modifying the RC network values at the output pins.