Help with "music trigger" for a DIY power amp

[mcdn]

I want to add a music detect feature to my DIY TPA3255 power amp. Idle power is low but at 8 channels x 260W/channel it does add up. I'm using a Neurochrome ISS from @tomchr to control startup, and it has a trigger input perfect for this purpose - all I have to do is short it to ground when there's music playing.

Now Google is surprisingly useless at providing me with sensible circuits for this task, or maybe I'm using the wrong search terms. @tomchr was kind enough to send me a schematic he designed, but it has a fairly high component count and also would need adapting to work off a 5V supply, which I would no doubt do wrong. It did however set me off in what I think is a workable direction...

The thing is I'm not sure if I'm doing it right. Frankly I know just enough to be dangerous around analog electronics, being a computer person rather than a solder person at heart.

Anyway... here's the schematic I've come up with. It only uses a dozen passive components and could be built onto a tiny board using hand-solderable SMD parts. The flow is basically:

1. Instrumentation amplifier as a buffer, amplifier and balun. Input impedance is in the gigaohm range. This gives an amplified single-ended version of the input, regardless of whether the input is balanced or SE. Gain is set by a single resistor, which at 1k1 ohms is about 40dB or 100x. This should saturate the output for most music signals
2. Smoothing cap to er, smooth the output signal
3. Comparator to detect "music signal" - output goes high when smoothed output from the buffer is above a voltage reference (the voltage reference is just a resistor divider between Vcc and GND)
4. Edge conversion so "music stopped" is a pulse input to the 555 timer (a series cap and pull-up resistor)
5. 555 timer to produce a high "run on" signal for 10 minutes after music stops
6. OR gate to send "amp on" signal when either music is playing or the run-on timer is active

This seems to work OK on a breadboard (although I don't have the INA126 yet), but I'd be very grateful if anyone can point out fatal flaws before I do something crazy like lay it out on a board and actually build it for real. There are clearly a few parameters that can be tweaked: input gain, smoothing cap value, reference voltage, 555 RC choices. There might also be some "NO DON'T DO THAT IT WILL CATCH FIRE!" errors.

EDIT corrected some silly errors on the INA126 connections and the value of C2