Each of the 5 vowel letters represents 2 different vowel sounds, called "long" and "short" even though they are different sounds, not different lengths. English spelling uses "silent E" to write the long vowels: vowel+consonant+E = long vowel.
For example "mate, rate, sate" all are long A sounds (/ei/ diphthong) while "mat, rat, sat" are all short A sounds. The silent E is kept before the plural S, so "mates, rates" are long A while "mats, rats" are short A.
So "quizes" would be the plural of "quize" (long I in both words), while "quizzes" is the plural of "quiz" (short I in both).