There's an old saying - Whenever you've eliminated everything else as an explanation, whatever is left , no matter how unlikely, is the explanation. So what can we eliminate?
You're running the exact same XP SP3 install with no new Windows updates, right?
You've installed again the exact same FSX Standard disks with the same SP1 and SP2 updates, right?
You have no speakers at all and you simply have replaced your headset, right?
Just as before, you hear every sound from every source other than FSX through your headset, right?
In fact, in addition to all ambient sounds (engine, wind, etc.), you even hear voices in FSX (ATIS and ATC) through your headset, but the voice playback is messed up, and this was so after you replaced the headset and then occurred both with your previous FSX installation as well as with a fresh installation, right?
If all that's right, the only fresh variable is the new headset - unless the trouble with the old headset is relevant. So let's focus on the headset.
What kind did you have before, what exactly was the trouble that arose, and what kind do you have now; what kind of connector was used by the two different headsets (direct to motherboard backplane audio connector, or USB, or whatever); and what is the selection you have specified in the FSX Settings> Sounds> Voice for the Voice playback hardware - Default or Realtek - and have you tried switching it from one to the other? If you have USB headphones, there may be a third option for a USB device, in which case you should try all three options. Have you checked the Sounds Control Panel settings to be sure your "Speaker" setup is still designated as stereo headphones?
I finally got a reply back from Microsoft. While I did remove and reinstall the codecs, I didn't properly rebuild them. So here, for the sake of seeing the solution is what Microsoft suggested:
To install a Microsoft ADPCM Audio Codec missing codec on your computer, follow these steps:
1. Click Start, click Control Panel, and then click on Classic view
2. In Classic View double click on the first icon, Add Hardware.
3. Click Next.
4. Click Yes, I have already connected the hardware, and then click Next.
5. In the Installed hardware list, click Add a new hardware device, and then click Next.
6. Click Install the hardware that I manually select from a list (Advanced), and then click Next.
7. In the Common hardware types list, click Sound, video and game controllers, and then click Next.
8. Under Manufacturer, click (Standard system devices).
9. Click Have Disk.
10. In the Install From Disk dialog box, click Browse, and then open the following folder: C:\Windows\System32
11. Click the Mmdriver.inf file, and then click Open.
12. In the Install From Disk dialog box, click OK.
13. Click Continue Anyway if you receive the following message:
The software you are installing has not passed Windows Logo testing to verify its compatibility with Windows XP. (Tell me why this testing is important. Continuing your installation of this software may impair or destabilize the correct operation of your system either immediately or in the future. Microsoft strongly recommends that you stop this installation now and contact the software vendor for software that has passed Windows Logo testing.
14. In the Model list, click the codec that you want to install, such as the Microsoft ADPCM Audio Codec, and then click Next.
15. Click Next to start the installation of the codec.
16. Restart the computer after the codec has been installed.
Try to test the game.
And I tested it and it's fine. Now the only problem is I lost all my tweaks, addon aircraft, and scenery. So now it's on to a slow rebuild.
/me
---ND6849C using FSX Standard on an Insignia D400a Pentium 4-2.8mhz-2 gig RAM-256MB under Windows XP (SP3) using an ATI Radeon 9250 graphics card.
