Since you have proven that the processor does not seem to be getting hot enough to legitimately trigger the fan's high speed, I don't see what good undervolting the processor would do. I could see slowing it down if the temps were getting high, but that does not seem to be the case here based on your info...
As DVaD posted, have you checked to see if there is a newer BIOS available for you machine?
I am confused that at one point you said you changed some BIOS settings, but then you say that Sony does not allow you to change them. Can you clarify that any more?
Also, just for verification, based on the post I am guessing that the fan has not always run fast since you got it, but is a recent symptom. Is that correct?
Can you think of anything at all change around the time the symptom started? An update or upgrade to software/hardware, lighting storm, etc?
Was the fan running on high all the time the only reason for replacing the fan originally, or was there something else?
Just fishing for clues here... kind of an odd problem...
Lastly, I don't remember as it's been a while since I have worked on a Sony, do they provide any diagnostic utilities? If they do, it might be worth a pass to have them check the temp sensors and fan speed controls and such. I mostly work on Dells, and I know their diagnostic utilities test those things, although I've never seen one fail yet...
I never did change the BIO settings. It is not possible. There are no updates for the BIOS available.
The fan has always run at high RPM since I bought the machine a year ago. I took it back to Sony. They replaced the fan.
I don't know of any Sony diagnostic utilities.
I have a feeling it is a BIOS issue since when it comes out of sleep, the fan resets to the RPM when the computer is initially booted.