Have you considered giving a spelling of the before and after progression of the sounds with the international Phonetic Alphabet (IPA)? Singers in particular, but also actors and other elocutive professions sometimes use IPA. I'm a classical/operatic singer - we use it mostly because the number of languages we're expected to be able to enunciate with nuance and act in are much greater than the number of languages you can learn while you're studying music (not an easy degree course, even without the languages). So we write the correct pronunciation in IPA below the music and memorize it so there aren't any questions about where to put the liason or if that vowel is mixed or not in Norwegian etc. et all.

IPA is fairly comprehensive, I don't think I've bumped into a phonium I couldn't express relatively accurately with IPA. Exampl: 'Fleur,' in French might be tough for some novice American pronouncers because the mixed vowel doesn't exist in our language, so I might express it to a student as [flœr] so there's no question about which mixed vowel to use; or to clarify to a student that the vowel in 'bacio' is not as "dark" as the Californian pronunciation of 'father,' say, I would spell it [batʃo]. This is distinct from phonetic spellings you might find in a dictionary. I often find those are short-hand versions and therefore less precise.

I don't know if that helps for your purposes, but I've linked to some very basic pages with charts and slightly more in depth explanations (one regarding French, since that was my first example) in case you can use it!

A good guide to IPA we use din school (and slightly more authoritative than the following links): International Phonetic Alphabet for Singers: A Manual for English and Foreign Language Diction, by Joan Wall

https://www.thoughtco.com/understanding-the-french-language-using-ipa-4080307

https://en.m.wikipedia.org/wiki/International_Phonetic_Alphabet