If some narrations in your PowerPoint:mac presentations don't play, that might be because they somehow ended up being Unix executable files! Yesterday, I got this client presentation that had all the narrations, but PowerPoint would not play them because they were not the AIFFs that PowerPoint was expecting. So what is the solution? It's not too difficult -- you'll need to download a free copy of Audacity for Mac.
Open Audacity and choose Project | Import Raw Data.
This will bring up the Finder -- navigate to your Unix Executable file and click Open.
Audacity will bring up the Import Raw Data sheet -- make sure you select the Two channels (stereo) option and set the sample rate to 44100 Hz. Then click Import.
Audacity will recreate your sound from the raw data -- hear it to confirm that this is exactly what you need, then proceed to save it.
Choose File | Export as WAV and overwrite the existing Unix Executable file (do make a backup of that file first as a precaution).
Audacity will add the WAV extension anyway -- you might want to remove the extension in Finder and check if the narration plays well inside PowerPoint. If it doesn't, just delete the sound icon on the slide that contained the narration, and re-insert the narration using the Inserts | Movies & Sounds | Sound from File option.