Distorted Audio Coming from Speakers and Line-Out

I've been working on my early-2008 iMac of late, replacing the HD with and SSD, and upgrading the CPU to 2.8 GhZ. The SSD and new CPU seem to be working fine, except after I finished I ended up with this weird audio problem: ALL audio is distorted. I've shared a couple of links to demonstrate:

Clip of Middle C

Stars and Stripes Forever

This happens both through speakers and audio out. I cannot seem to find the source of the problem. I've tried the following:

• Replaced the audio board
• Replaced the audio board / logic board flex cable
• Replaced the logic board
• Booted off an external HD
• Downgraded to Mavericks
• Reset the PRAM
• Reset the SMU
• Terminated CoreAudio

NOTHING seems to work. The audio just keeps coming through like it does above. My only thought is maybe my power supply is shorting out and messing with the audio, but I don't want to pony up for a new component until I have a better idea.

Does anyone have any ideas?

Note: This only happens after I boot. The startup chime is not affected.