This one's got a long history. Bear with me.
In January 2009, I bought a new MacBook Pro to replace the nearly-one-year-old MacBook I had. The white MacBook I had suffered from a myriad of issues, mostly due to crappy case construction. Having played with the unibody MacBook Pro's at an Apple Store, I decided to get rid of the MacBook.
I needed this computer for college, so I got it dual-booting Mac OS X and Windows Vista. Got everything completely set up and running.
That's when, having everything done and ready to go, I decided to listen to music.
I was listening through my iPhone's earbuds, and my roommate left a few minutes after. At this point, I decided to listen to the totally awesome speakers that come on the MacBook Pro.
Except, when I pulled the plug, there was nothing but silence. And the volume controls didn't work at all. And there was red light coming from the headphone jack. A quick trip to System Preferences revealed that somehow, my computer was stuck on Digital Out.
Now, this was a known issue in the older MacBook Pros, but I had never seen it on the new unibody MBP's. So, I tried every arcane fix that worked for the other MBP's. Nothing worked. Sensing defeat, I finally just packed up the MBP in it's box and exchanged it on the recommendation of AppleCare.
I had to leave town with my (then) girlfriend, so I took it with me, unboxed it at her house, and set to work on it. Once again, I plugged my iPhone earbuds into the computer and listened to music. Once I pulled them out to listen to music through the speakers...
BOOM. Dead. Again. Red light shining bright through the jack, and the computer is jammed into digital out again.
Naturally, I am pissed. I call up AppleCare again, and explain myself. The kind guy on the other end concludes with a "well, I think you pretty much know what you're doing here, so what would you say would be a good solution?"
I just requested to get another exchange, but at a different Apple Store, because the batch at the store I purchased my MBP from might be defective. He added a note, and sent me to the Apple Store San Francisco.
I got there, they tried to revive my computer, and upon failing, swapped it out with another MBP. This time, I plugged and unplugged my iPhone earbuds into the MBP to make sure it didn't break again.
It didn't. And it still hasn't. Sort of.
Fast forward to about a month ago, in which I install 10.6 on my MacBook Pro. From this point on, the sound card seems to be making this weird pulsating noise in the background of every quiet song that I play.
Here's the problem in action. And it was tested against multiple devices.
I learned to deal with it for a while, but it's beginning to get on my nerves. So I set out to fix this issue.
Firstly, the diagnosis reveals that this problem only occurs with headphones. I tried to hook the headphone jack to my Eee PC's Line In jack, and while the sound makes it across clearly, the pulsating is not present. Also, bumping the frequency to 90,000KHz in Audio MIDI Setup made the problem way worse. And also very documentable.
As for actually fixing the problem: My first shot was to reboot my MBP in Windows 7 and play the same song. And it played, no pulsating whatsoever. Screwing with every setting I could find didn't make things worse, either.
Okay, so this has to be a driver issue of some sort. Only...you can't install/reinstall drivers in Mac OS X!
Remembering OS X basic troubleshooting steps, I tried playing the same song in a different account. Problem persisted. So obviously, something is screwed up in 10.6.2.
Getting on the extreme side of things, I installed 10.5.5 to my external OWC drive. With a stock 10.5.5 install, I played the file...and, holy shit! No pulsating!
I began moving all my critical stuff from my MBP's drive to the external drive. Once everything was migrated and in place, I used SuperDuper to copy everything back over to the MBP's hard drive. Rebooted it, and everything was still fine.
Pushing my luck a bit, I let my computer update to 10.5.8, but I had it leave QuickTime and iTunes alone.
And, surprise surprise, the audio was once again broken when I tested it under 10.5.8.
I reformatted my MBP's drive and restored everything from the external disk, which still had 10.5.5.
Everything works great.
And I'm not sure I'll ever touch Software Update. At least, not until this issue is fixed.
--------
PS: If anyone wants to have a crack at diagnosing my problem, here's my system info (1MB file). I can grab console logs and reboot into 10.6.0 to get a profile there, too.