My internal microphone dead after replacing hard disk

Hello,

I fix computers for a living, and with Macs, one thing that frequently needs replacing is the hard disk. No problem....get a new one, carefully replace it using the excellent iFixit guides (which I know by heart now....) and then install Mac OS X, migrate and voilà, good as new!

Almost.

I've had a number of people tell me that the microphone is no longer working. The thing is, and I've checked again and again and again, I never touched the microphone cable. It's very well hidden behind/under the iSight camera (and ambient light and camera "on" indicator) and the cable snakes around behind everything and is under tape and so on.

So, now I really worry about replacing hard disks. Such a simple procedure, but if they're gonna lose their microphone... not good.

I really have no idea what it could be..... Any ideas? Better yet, any solutions?

Please help.