I'd like some ideas for adding notation to a piece of choral sheet music for the benefit of unification of vowels, diphthongs, and consonants. We already show breaths with a " ' " and no breath with a " n.b. ." I need add notations to show choir members how to pronounce a vowel, when to turn a diphthong, how to pronounce an ending consonant at the beginning of the next word. Any ideas?
Ultimately, the only 100% (well, 97.6%) objective way to indicate this is with the International Phonetic Alphabet. Most singers that have trained professionally as “classical” singers will have some familiarity with these symbols, especially those that symbolize phonemes common to the Romance languages and English and German. Even linguists will be unfamiliar with some of these symbols and would need a refresher.
Off the top of my head, the main scores I know that use IPA consistently are Luciano Berio’s Sinfonia and Gyørgy Ligeti’s pieces Aventures and Nouvelles Aventures. George Crumb uses a sort of quasi-IPA in several of his pieces. Those examples are highly complex, but the exploration of unusual phonemes are entirely the point for those works. In more simple use, these symbols are effective and shouldn’t take the singer long to pick up.
At least in April of 2018, this Wikipedia page is informative and fairly comprehensive. However, the most important symbols for vowel specification can be summarized here. The symbol [i] stands for the vowel sound in the English word “bee.” The symbol for the initial vowel in “may” is [e], although already we start to glimpse the rabbit hole that could await us: in most accents of English, the vowel is actually a diphthong that moves quickly from [e] to [i] or [ei]. The vowel in bet is [ε], the vowel in “cat” is [æ]. “Boot” is an [u], “boat” is pretty close to [o]. The sound in most American accents of “cot” is [ɑ], which shouldn’t be confused with [a] which is more for a sound very similar to [æ] but slightly more open (like some southern US accents on the word “hat”). [ə] is the neutral vowel sound kind of like “uh” where the tongue is in the middle of the mouth. Most vowels in American English have become this default “uh” sound, although it tends to be avoided when singing. Anyway, you can and should explore all of the primary vowels, but those are the most easily recognized for English sounds. German ü sounds are symbolized with [y] and German ö sounds with [ø].
If you don’t want to use IPA, then your best bet is to specify what language, dialect and accent you had in mind, and write your lyrics as consistently as possible.