Edit the narration for a slide after it has been recorded
You've used PowerPoint's Record Narration feature to record sound for your slide presentation. Now you need to change the narration for one or more slides, but you don't want to have to re-record the entire narration.
If you linked your sound files when you recorded the narration (it's a checkbox in the lower left corner of the Record Narration dialog box) then each of them is a separate file on the hard drive, probably in the same folder as your PPT.
Work out which file goes with which slide, record a new narration using eg. Sound Recorder or the free Audacity sound editing program
Once you have a sound file you like, save it to the same filename as the original sound file for that slide and you're good to go.
If you didn't link, there's another step or two.
Save the presentation as a web page (not Single File MHT web page). This creates an HTML file plus a folder of support files by the same name as the base HTML file.
In that folder you'll find sound files, one per slide. From there, follow the same instructions as above to record a new sound file and save it to the same name as the original for the slide you want to correct.
Finally, use File, Open from within PowerPoint and open the HTML file you saved as a web page. It'll open back into PowerPoint as a presentation. Test it and if it's ok, save it back out as a PPT (presentation) or PPS (show) file.
And in either case, linked sound files or not, if you're good with Audacity or another sound editing program, you might be able to use it to edit the original sound files, removing problem sound, adding new sound and so on.