Where the Audiobook of Dark Sun’s Prism Pentad doesn’t give me a direct pronunciation, I fall back upon the closest real world language which fits that particular word. In this case, it would be Greek:
I’ve been saying it THRY-kreen since the original box set came out in the 90s not sure how my group got pronouncing that way. Maybe because the “i” in thri should be said different than the “ee” in kreen.
I’d say that the final “s” in Arisphistaneles is pronounced as a voiced alveolar fricative. Meaning it sounds like a “z”.
In English written s’s at the end of words can sound like a “z” especially if the preceding consonant is voiced. “Pips” vs “pins” or like the case you mention it is preceded by a vowel sound. In this case the long e. But there are other rules for s that don’t always follow this.
I’d say the ending sound is like “leez” similar to the word “bellies” (plural of belly)
The first written mention of the pronunciation of the word “mul” was in the DS box set pg 14 of the Rules Book. The original printing of the DS boxed set had “mul” with a bar over it, indicating long u. Not the umlaut (two dots) over it as seen in the OCR digital version.
The one problem with the “long u” in English is that there can be two pronunciations. “Yoo” as in “fuel”, “mule”, etc and “oo” as in “cool”, “tool” etc. which would give you pronunciations like “mule” for the one or “mool” for the other.
That said, either one of the above could potentially be correct. But back then, the short u as in “but”, “mull”. Would not have been considered per the provided pronunciation.
I think the original authors struggled with the word as seen in the various typos when writing “mule” instead of “mul”
Things changed 19 years later, in 2010, with the 4e release of DS. There they included three different pronunciations of the word and included the short u this time “Mull”. The authors even went so far as to say the short u version was the most accurate. Despite 19 years of previously established pronunciations that did not include “mull”.