After a fierce battle, I was finally able to restore sound to WitPAE!
The short of it is that my hunch was correct - something had change with regard to DirectX in the 6 months or so that I wasn't playing the game. There were too many patches that rolled out to know exactly which one did it, but I used a utility to "roll back" DirectX to the original settings, and - voila! - WitPAE was back to its usual self.
So for any of you who may find yourselves in a similar situation in the future (i.e. audio/visual problems that can't be explained by missing files, incorrect installation, etc.) -
The issue will probably lie with DirectX in some way.
Microsoft has not made it easy to uninstall/reinstall or otherwise maintain DirectX. About all you can do via Windows itself is install DirectX if you don't already have it. And with Win XP or after, you will most certain have a version of DirectX - you just may not have another version which may be required to run a particular game you are interested in playing. The latest versions of DirectX (such as DirectX for Windows 10) can only be obtained via Windows Update - and if Windows Update thinks you already have the 'latest' version, there is no easy way to reinstall it to check for any corruption!
After a lot of research, the best utility I found to deal with DirectX is DirectX Happy Uninstall. The good news with this utility is that you can use it to backup your current configuration of DirectX, roll back to the 'original' DirectX that came with your version of Windows, "force install" another version of DirectX you need, and even uninstall DirectX (which may cause unpredictable behavior on your machine). The down side is that it costs $20.
I went ahead and purchased the utility myself - but it only halfway helped. What I mean is that I was able to back up my then current DirectX implementation and then roll it back to the original implementation; therefore, I was able to observe that something in DirectX was the cause of my audio issues with WitPAE. The only problem was that my Windows start button no longer worked! So I could either have a working WitPAE or a working start button. Not good...
So how I fixed *everything* was to repair my Windows installation. There are various ways to do this, but the most straightforward and *complete* way I found is to use the Windows 10 Media Creation Tool. It's a utility you get directly from Microsoft which allows you to create Windows 10 installation media, meaning a DVD, USB drive, or even a direct download to your hard drive that can be used to install or upgrade Windows 10. The trick is that you can still upgrade Windows 10 even if you already have it! Effectively, you are doing a complete repair install by doing that. The utility makes it very easy, going step-by-step, and the best part about it is it's free (assuming you have a valid Win 10 license). And don't worry, you can tell it to keep your documents and programs just the way they are. The only thing it changed were some audio/video configuration settings (like how many audio channels on the speakers, color calibration on the monitors, etc.). But those are easy enough to tweak again.
Before I go, I want to mention two more things you can try *before* resorting to a full Windows repair/upgrade using the Media Creation Tool. From the Command Prompt (with administrator privileges), try typing in the following command:
sfc /scannow This will cause windows to do an integrity scan of core files - it may be able to fix whatever the problem is right then and there. Keep in mind it takes at least a few minutes to complete, but you can do other things while it completes.
The next escalation is to use the command:
DISM /Online /Cleanup-Image /RestoreHealth Also from the Command Prompt w/ admin privileges. This will download a source image of the Windows installation disk (or equivalent) and use it to repair corruption it finds in your current installation. Think of it as a "repair medium" if the sfc scan is the "light" and the Media Creation Tool is the "heavy".
If neither of these work (as neither did in my case), then it's time to use the Media Creation Tool for a complete repair reinstall and/or upgrade. Finally, if you don't know how to open the command prompt with admin privileges, just press Crtl + Alt + Del on your keyboard to open Task Manager (for Windows 10, left click on the last option in the list that's presented). From Task Manager, click on File > Create New Task. Type in "cmd" (without quotes) in the box, then check the box next to "Create this task with administrative privileges". Then you'll have your command prompt window to follow the instruction above.
In any case, hopefully this might help someone have a few more options when diagnosing problems with WitPAE. Good luck!