I have an edition with a pronunciation guide. I don’t have it at hand, but I recall being surprised by the pronunciation of Eilonwy, which is “AYE-lon-whee”
Shouldn’t Eilonwy be pronounced something like AIL - un - oo - ee, since it’s a Welsh name? And Arawn would be a - ROUN (like “round” without the D).
(Not to dispute your pronunciation guide, Hello Again, since the author’s word is authoritative. And my grasp of Welsh pronunciation is tenuous at best. Just saying, is all… )
This page supposedly shows the pronunciation of most of the main characters/places. However, this might just apply to the movie and they very may well have butchered some of the Welsh.
So shoot me. It’s the best I’ve been able to find so far! They even link to it off of Lloyd Alexander fan pages, so there must be a couple correct statemements in there somewhere.