What a frustration. But, after having this happen a couple
times, I found a remedy:
When publishing, presenter creates a file, data, in the same
folder where you published. Withing the folder, data, you'll find
all of your audio files, named, for example, "a24x1x1", "a24x2x1",
etc.
Here's what you do: Using "Insert" from the PowerPoint menu
bar (not Presenter menu, but PowerPoint menu; important), insert
each sound into its respective appropriate slide, one by one;
selecting "Automatic" when asked when the sound should play each
time.
When you are done inserting all of your sounds, now go to the
Presenter options. From their click the import sounds icon (near
the microphone button). When the dialogue box opens, choose "Import
sounds from slides". Click Okay in the next dialogue box.
That's it! You tricked Presenter into thinking you were
importing embedded sounds from the PowerPoint when, really, it was
importing the sounds it had made itself, earlier.
It's tedious, I know. But not nearly tedious as re-recording
hours of audio and voice.
Cheers!