So Norse/Celtic mashup? As opposed to Greek/Roman, Egyptian or further afield?
There are very few written Celtic things so you might have a harder time there if you want to go "accurate", but there are some efforts otherwise (mostly going to be Irish takes rather than Scottish*, and next to no chance of Cornish, maybe a few Welsh but that gets very muddled. Anything Gaulish or Iberian peninsula is almost certainly going to be reconstructed).
For Norse stuff you can probably find someone read the Prose Edda (Icelandic but most of what current Norse mythology tends to be reverse engineered from as a fair bit got lost during Christianisation**, and generally considered fairly true to things) but I don't have a specific audiobook there.
*Scotichronicon and then mumble mumble mumble Tales of a Grandfather from early-mid 1800s tending to be how that one goes.
**indeed a fair bit that is not Prose Edda coming from monks sent to convert people noting down what the filthy pagans and heathens believed with all the distaste and lack of objectivity you might imagine there with the remaining being a few oral traditions.
If you did want the Greek (things are rarely covered from the Roman aspect these days it seems, though Roman=Greek with different names for the most part) then you can have short form and long form versions depending upon age bracket (kids stuff tending to be shorter, and not always sanitised which is nice). Fewer do Egyptian but it is popular among the kids so you can find it.
There are also likely to be about 5000 readings of Beowulf (it is popular among students of English literature usually because it is old rather than any great fondness for it if my experiences are anything to go by, and that usually translates as a bunch of said students giving it a go) if you want that. That is less gods and such but still foundational mythology. Likewise if you expand Greek and Roman beyond basic origin stories of gods and short stories involving them (see stuff like the Minotaur) then you will likely find plenty of readings of the epic poems which are far better than any filmic take I have ever seen of them (I have never seen a film involving the siege of Troy, which you will likely encounter in the Iliad and Odyssey for the Greek side and Aeneid for the eventual Roman one -- Rome in myth being founded by the survivors of Troy, that I consider anywhere near as good as the works mentioned in the aside) and still feature plenty of gods and mythology.
Going further afield you might get a few Indian things and then have to hit Japan*** before it really picks up again, with maybe some enterprising type doing Thai or something.
Middle Eastern is going to be fun -- I do recommend Arabian Nights (think Aladdin not the Disney version, and that can also go for the Chinese telling if you want). Might do something for Code of Hammurabi (technically a legal code but short and features some gods give me authority/stories thereof) and other things written in cuneiform or that fall under Assyria and Mesopotamia.
***Japan and audiobooks is an odd one in that they are not as popular as some might imagine. Indeed if you did want Japanese folklore then there are a variety of truly fantastic short story anime covering things here. Folktales from Japan probably being the main effort there but there are lots of others.
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Psionic Roshambo:
