Adding narration to PPT that already has existing audio/SFX

I have an existing 26-minute PPT that has 145 slides, embedded music, video, sound effects, etc. (It looks like an amateur Ken Kurns documentary). I usually do live narration to this when I present for historical groups, etc. I want to create a packaged version to play so that I don't have to narrate live.
Is it possible to overlay a new narration on top of existing audio and save it as a new Powerpoint with all of the old and new audio embedded so that it can be played without me being there? Essentially I want to run the PPT on my laptop and narrate it as if I were presenting it, then save the new file. I hope I am explaining that clearly.