[Quoth]

Originally Posted by LostOnTheLine

I mean it's been awhile, but I studied European Fae lore once upon a time but the Sidhe were little people.

No. Not in ANY old Irish or Welsh MSS. The little people idea is like 400 or 600 years later, or more and in English via England from the Continent, maybe as late as 16th C.

Originally Posted by LostOnTheLine

For the use case I was referencing Audio Books wouldn't be a problem because you'd just have a different reader for the different Narrator/Font.

That's audio drama, not an audio book. The production costs are like x100 more!
However some narrators are pretty good at voicing the characters, as has been mentioned.

Edit:

So this story is one that is one of the "Changed in the name of Christianity" stories that we don't even know what the original story was. The Catholic church sought out & destroyed a lot of Fae Lore & replaced a lot with Christian-ized versions of them.
Interesting off-topic comment: the pointed ears of Fae creatures were originally "leaf-shaped" & it is debated, but more accepted than not, that the leaf-shaped attribute was added by the Catholic church because it was a symbol of deformity that was associated with cursed, demonic, & evil beings.

I don't know what tangent you are on, but mostly in English and entirely in Welsh and Irish it's not true.
The old Irish stories and Norse stuff (via Iceland about 400 to 600 years later) have no mention of pointy ears.
Also best scholarship suggests the Irish monks writing down oral tradition hardly changed or left out anything, they did add things. They changed Tuath Dé (tribe of God) to Tuatha de Danu (Tribe of goddess Danu), but that's because in Irish the Hebrews were the Tribe of God. Danu wasn't an Irish Celtic goddess at all like the ones of Erne, Boyne, Bann, Shannon etc, but of the Danube (Severn & Avon Celtic goddesses got Romanised). Before the Romans destroyed most of it there was a Pan European Celtic culture. The Helvetii fled Slavs and about 70% killed en route to Switzerland by Romans. The oldest Celtic writing uses a mix of Greek, Etruscan and Roman letters and about 700 BC found in Switzerland. One of Rome's early famous playwrights was an Italian Celt.
You can see huge bits of bronze age oral tradition in the oldest Irish MSS.
The Book of Invasions certainly has Christian elements added.
I don't see a huge Christian revisionism in Thomas the Rhymer or the Nordic Sagas from 13th C. via Iceland.

Fonts
I've already agreed that this particular ebook needs the fonts but having encountered this before I know the author could have done it differently. It's either ignorance or egotism.
Fonts may not be small, though you can subset.
Also such a thing mostly only works in Roman-Latin fonts. It's not going to work in any Asian language and possibly not in Farsi, Hebrew, Arabic, Greek, and Cyrillic based languages, though I'm not sure.