Wow, judging by the massive amount of users contributing to that thread, it must be a widespread thing. I still think it's a hardware problem with that specific machine and not a Leopard/Snow Leopard problem.
Now, if it were a software problem, why would it start popping as soon as you turn the computer on (before the OS is even loaded)?
I've owned many, many Macs and seen many, many, many Macs and I've never run into a "popping speakers" problem.
I'm not saying that your problem doesn't exist, but maybe you should be bringing your Mac to Apple and have them warranty repair it, or think about getting a new computer, because it's not a software problem.
He could try booting into windows and see if the problem is still there.