Peculiar cupcake/OS problem

[midwayfair]

I had a perfectly functional Cupcake for days until I added the LED. The LED came on but now I don't get any sound. I very briefly had some heavily distorted sound at first, but that went away. I removed the LED and there's still no sound at all now. Very odd.

The transistors and IC are working, as is the switch and harness - I moved them to another completed board and everything works there. The protection diode only passes signal toward ground, so I know it's not burned out.

What else can have gone wrong? Maybe a blown electrolytic? Does anyone have the proper voltages at the test points? (And how do I test if a capacitor went wrong?)

Edit:
(1) Project Name: Cupcake
(2) General description of the problem: No sound after installing LED, or after removing the LED. Worked before installing LED.
(3) Steps that you have taken to try to resolve the issue: Removed board from harness, reflowed solder on a few parts, swapped out chip and transistors, and checked for continuity/voltage/resistance at all parts. No solder joints found.
(4) List any substitutions you used for parts/values: I subbed in a 150K for the 100K, a 1N270 for the 1N34a, and a 1uF cap where the 4.7 connected to them would have gone. (These are confirmed acceptable mods and worked prior to installing the LED.)

[madbean]

I would audio probe the circuit to see if you are getting any signal through at all, and at what point it is dropping out. Check these points:

R1/C1 for input.
Pin3 and Pin1 of IC1.

You should also see the following voltages:
9v on pin8, 0v on pin4, 5, 6, & 7, and maybe between 5-6v on pins 3, 2 and 1.

[midwayfair]

Hrm, I had 9v on some of the pins, 0 on a few others, and none were 4-5 ... also, I burned the volume pot (like, "sparks, fire, and smoke") while testing, so I didn't get a chance to write them down ....

There was no signal at C1 on the audio probe (?!) - even after replacing that cap.

I was going to replace the power filtering caps, too, but at this point, I'm just going to wait until the cupcakes are back in stock and I'll rebuild it. There's clearly something very wrong and the board's getting pretty chewed up from pulling parts in and out. I was able to salvage the stuff that actually costs more than a few cents.