Lisa, you've not made it easy for yourself.
The big problem you will have is one of synchronisation.
If the Fade Slideshow is playing like a video then make it into a video and embed the narration into that.
Then as you loop the video you can have a Narration On/Off feature that it properly synced as you've made it so.
Any other approach assuming no loading delays, no slideshow pauses or resets and no jumping between variants.
You have yet to cover how your quiet narration will sound on a vast array of systems with different settings. That omission is scary.
With a fair bit of coding I could probably manage a trigger per slide transition to start an audio as the correct point and stop on slide transition out to await the next trigger.
I welcome anyone's suggestions here.
Have a look at Google Slides.
It allows adding Audio to a Slide. I believe you can use the one audio file with different start and stop points for each slide. If not, use separate audio files.
You then use File > Share > Publish to web and in the Embed, tick Start Slideshow (autostart) and Restart (loop).
I have not fully checked but the audio engages without user interaction (
https://docs.google.com/presentation...3V4Tk/present#!).
This is an old approach as GS not directly includes Audio files (MP3) so not MP4 conversion is needed.
You would then add the Embed Code into a Placeholder -
Google Slides - Autostart & Loop.xar.
Acorn
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