It is possible that there are multiple pronunciations of the word "Terabithia"; however, I have only ever heard it pronounced one way. The "ter" section of the word pronounces the vowel in a way that would rhyme with "bear," "care," "tare," and "tear" (as in a rip in a piece of paper). The "a" after "ter" is pronounced with a short "a," which would sound like the filler word "uh." "Bith" pronounces the "i" in a the same way that it would be pronounced in common words like "it" and "pit." The "ia" at the end could be confusing. Each letter is going to be pronounced in the word, so those two letters each have a syllable in the word. The "i" is a long "e." This makes the double "e" sound like in the word "bee." The final "a" of the word is pronounced exactly the same way as the previous "a."

"Terabithia is pronounced with a short "e" sound as in the word "bear", in "Ter", and the short "a" sound as in "about" for the "a" and a short "i" sound as in "inch" for "bith" and finally a long "e" sound as in "ease"  and then another short "a" sound as in "about" for the "a" at the end.  I know this looks elementary and complicated, but as there is no dictionary respelling, this is the best explanation I could give.   I also put this together

(těr uh bith ē uh)  I hope this helps.