These pronunciations come from The Prydain Companion: A Reference Guide to Lloyd Alexander's Prydain Chronicles by Michael O. Tunnell. In The Prydain Companion, Mr. Tunnell writes:
The pronunciation aids were provided by Lloyd Alexander, who advises us that they are not necessarily true to the Welsh tongue. Many sounds in the Welsh language are simply not available in English. Alexander (1985b) does not want the Welsh words to be a stumbling block for readers of the Prydain Chronicles and has therefore made the pronunciations as simple as possible.
While putting together this visual, I have worked closely with Mr. Tunnell to decide which names to include and which names to leave off. I didn't want this graphic to be a comprehensive list, whereas The Prydain Companion already serves as such. However, hopefully this will be helpful to anyone who enters the land of Prydain and is having a hard time with some of the Welsh names. As you come across names that are not on this list, please refer to The Prydain Companion for a complete pronunciation guide.