Microphone volume suddenly dropped by about 75%--tried everything, need help!

Up until a few days ago, I was using the microphone just fine for both instant messaging and voice recording, with very good volume, and then, all of a sudden, the recording levels dropped by about 75%. Now, when I record normal speaking voice 4 inches away from the mic, it sounds VERY faint on playback (-30db, according to Adobe Audition). Also, when talking on Yahoo Messenger or Skype, the other party can barely hear me, forcing me to practically have to swallow the mic to be heard.

I've tried everything I could think of to fix the problem: checking the volume setting (it's set all the way up for the microphone, and the mic input IS selected), reinstalling the audio drivers, even reinstalling the "notebook system software" driver and the chipset drivers, all to no avail. The microphone boost is set to OFF, but then it has always been, since I don't appreciate the high noise floor that it produces on recording. The microphone itself is not the problem, either. I've tried two different mics--with the same result.

My system is an Inspiron E1505 running Windows XP Media Center 2005, currently with 4GB of RAM, and a 160GB hard drive. The audio adapter is an on-board SigmaTel STAC9200 (driver version 5.10.0.5515).

The only thing that had changed in the system between the time when the recording worked and when it broke down was that I expanded the memory from 1GB to 4GB. I know that the E1505 is only rated for up to 2GB, but I did my research and found that it WILL take up to 4GB without any problem at all, just that not all of it will be recognized (in fact, only about 3.3GB is now being shown as available). The memory upgrade went off without a hitch, so I doubt very much that it could be the source of the problem. In any case, I've also tried putting back the original 1GB RAM, and the problem is still there.

I have run the complete set of system board tests (accessed by pressing F5 from the startup screen when the memory has been changed), and everything passed. The mic recording test, however, very clearly showed the same low recording levels, which to me seems to indicate a hardware issue, since the operating system wasn't even loaded yet at that time.

I have also tested the voltage on the mic jack, and it turned out to be 3V (as measured between the ground ring and either of the two channels). I've read online that it should be 5V. Could this be part of the problem?

I would appreciate any help and suggestions, as this issue is getting critical for me, especially because of the devastating effect it's having on my IM communications, which I am using for work!

Thanks!

qhn: Many thanks for replying.

I've tried recording through two different programs --Adobe Audition and Audacity-- with exactly the same result (low volume). On the IM side, I've been using both Skype and Yahoo Messenger, and again, the low volume is present in either case. It seems to be a problem right across the board, in all applications. I haven't tried uninstalling Audition, which was the latest sound-related application to be installed on the system. I could try that, but do you really believe it could be affecting sound globally even when it's not running, AND after all the drivers have been reinstalled (including the system software and chipset drivers)?

On the other hand, does the 3V mic bias voltage sound right to you? I've looked and looked online, and couldn't find any reference to what the voltage should be for the STAC9200 codec. I did see, however, that it has several "confiigurable" pins for VREF microphone bias. Unfortunately, nowhere does it say what the respective settings should be. Any ideas?

Well, I reinstalled the UAA Bus High Definition Audio driver, and the sound improved, so, problem solved. (Thanks, qhn for your suggestions!)

I'm a little confused, though: at first, I thought the UAA driver was a low-level bus driver, separate from the sound card, but when I read up on it, I realized it was actually a Microsoft-proposed (or IMposed) universal alternative to manufacturer-provided sound card drivers, which means that it is supposed to operate at the SAME level as the manufacturer's driver. Did I get it right, and if so, how can two different drivers handle the same device at the same time? And why does the manufacturer's driver appear as a sound device, and the UAA driver, only as a "system" device?