What are letters representing the sounds a, e, i, o, u, and sometimes y called?

The correct term for the letters representing the sounds a, e, i, o, u, and sometimes y is "vowels." Vowels are essential components of the English language, as they are sound-producing elements that can form syllables when combined with consonants. In English, these letters serve as the foundation of syllable construction, allowing for the formation of words and facilitating pronunciation.

Vowels differ from consonants, which are other letters in the alphabet that cannot stand alone as syllables and require vowels to create sound combinations. Diphthongs are specific vowel combinations that create a single, gliding sound but do not represent the complete set of vowel sounds in the language. The term "substitutes" does not accurately describe any established category related to sounds in the context of language and phonetics. Thus, understanding the role and definition of vowels clarifies why this is the correct choice in the question presented.