שמע shəma‘ - ‘listen’
שמא shemā’ - ‘perhaps’
The difference in pronunciation between ‘ and ’ is now lost, since both have become zero (not pronounced). The distinction between long and short vowels has been lost too. The only difference between the two words in Modern Hebrew would be the shwa in ‘listen’ vs. the vowel /e/ in ‘perhaps’. The dictionary says that shemā’ is a literary word for ‘perhaps’, it isn’t used in conversation, so its pronunciation matters less. If you look up “perhaps” English to Hebrew, shemā’ isn’t listed; instead it gives ula or yitakhen.
The only word I can think of that consists of a single vowel is the conjunction ve spelled with the letter vav, which is pronounced “u” in certain sandhi environments. That sounds as if the pronunciation changes when you go to the beach or the desert. “Sandhi” is a word used by linguists, referring to sound changes at word boundaries. It comes from Sanskrit roots meaning ‘put together’ and is a cognate of Greek synthesis.
Except that ‘ayin was a consonant in the ancient language. It still is in some Sephardic dialects. In Yemenite Hebrew, I think. I wonder how long after ‘aliyah it takes the dialects to die out?