When we communicate, ideas convey the message. But words are the building blocks that shape meaning. In reading aloud, what we read is predetermined by the thought vocabulary of the author — the way he chooses and uses his words. If the reader really understands the words before him, he can readily grasp the thoughts they form and hold to the meaning as he reads. The listener hears the message rather than words alone. But if the reader stumbles over words, the thought-patterns may be obscured. Both reader and listener struggle for the meaning.
What can the reader do with words he's uncertain about? He can research them in high-quality standard and Bible dictionaries, such as those listed in the resources section of the Handbook for Readers. Whatever source one uses, he needs to study the compiler's system of diacritical markings to be certain of the intended sound of the word in question.
There may be more than one acceptable pronunciation given. In that case it's usually simplest to choose the one that's most common in your locality.