My guess, at this point, is that the on-board audio adapter is defective. Could have been a short when connecting powered speakers or when you swapped your PSU. I suggest you disable the on-board sound in your BIOS and get a stand-alone audio card at this point.
WAIT! Before you do the above. Do you have a Linux CD or DVD by chance? If not, you can get one at :http://www.ubuntu.com/download
Burn the ISO to a CD and then boot from the CD (into the Live or Try It selection, nothing will be written to you HDD). If the sound is working properly, Ubuntu will generate sound when it starts up. This "test" can verify of this is a hardware or software problem.