Why Does a Circuit Stop Working...

I had a circuit developed and tested successfully. The circuit board actually had two circuits - an audio amplifier and an audio envelope follower - on it.
The audio envelope follower stopped working.
I did mount it in an enclosure.
The amplifier still works - so doubt if its a power problem.
Visual inspection of the board does not show any bad solder joints nor broken traces. It was a commercially prepared board (hence I don't think its a bad trace problem.)
Everything except the basic circuit has been disconnected, to eliminate any related issues.
What else can I do to check it out? Check resistors/trim pots with my meter... (in circuit?)... Check diode for function (in circuit?)... Replace Op Amp...
Any other ideas? (The circuit in question can be seen in my album)

I had a look at your design.
There is nothing to check but the OP-AMP.
Replace it and check.

Could the board have flexed while being mounted ? Only takes one hair-line fracture to spoil your day...

Blew it out somehow? Static electricity, ungrounded soldering iron, poor grounding making the supply voltages float, etc?

Nik - don't scare me! the board definitely acidentally flexed while installing...
biff - positive the grounding was fine; could've been static electricity and was going to replace the op-amps tonight...

However, it helps when you are processing an audio signal that its volume is high enough to be detected by the circuit as designed... My bad - I found out that my media player had its volume setting turned way down. Basically, I paniced because I felt that I didntt have the tools nor experience to debug the problem. Thanks all for your suggestions!

I'm *very* glad it turned out to be something so simply remedied.

FWIW, I thought I'd blown a 6n139 optocoupler in a breadboarded circuit, then found the LED's LM334 programming resistance was 1k8 instead 180 R. Not entirely my fault: Supplier had a picking error and I did not check value with meter or loupe...