Thanks for the inputs on this. I also ran into a problem with missing audio, and after a search on YouTube, I saw that this appears to be a fairly common problem. One fix appears to be:
Save your project
Close Vegas
Reopen Vegas
Reopen your project
“Save As” an MP4 without doing ANYTHING ELSE
At least that worked for me. Next time around, I will do some poking around to see if I can find improper settings.