NOTE (25 July 2009): has dropped the American Heritage Dictionary reference work that had the audio pronunciations to which many links on this page pointed.
The accent marks follow the syllables on which major emphasis should be put when pronouncing the words.
Aaron [Aar'on], Caesar [Cae'sar], Joshua [Josh'u·a], onycha [on'y·cha], papyrus [pa·py'rus], Sadducees [Sad'du·cees], shekel [shek'el], Zion [Zi'on].
Thus not all variations in vowel pronunciations are actual mispronunciations.
Consonants are a little easier, but some consonants and consonant combinations (such as C, G, CH, and TH) have more than one sound in English, and the differences can lead to mispronouncing a word.
The resulting pronunciation is often the same or nearly the same.