Ok bad news and great news. First the bad news, I wasn't able to play a replacement sound over the muted sound, which in retrospect is perfectly logical and reminds me not to experiment late at night.
The great news? I was able to replace the original game sound with my own clip effortlessly in a few seconds.
Again, thanks go out to @a63nt-5m1th for his export to Open Formats solution.
So here are the steps.
Once you have muted the wav file in Audacity as per the instructions in post above and saved it, you can now reopen it and give it a second pass if necessary to completely mute the file (select all, effects, amplify, slider to the left).
Now Import your replacement file (same wav format) and trim any excess because the files have to be identical in duration. Audacity draws a nice neat line where the file ends, so it's super easy to match them up.
You now have three tracks - the original muted wav from the game and two tracks from the replacement wav (unless you recorded the replacement in mono). From Audacity you split stereo to mono because you need a mono file, and save it.
As per the instructions above, you drag the oac back into Open Four. That's it, your done!
The video below was just a super quick and dirty proof of concept, so the synch is not there, but that was expected given I didn't make any effort to get a similar wav, just chose a random one, and a female voice for Michael! If anyone needs more details, just ask.
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