Vowels were not a part of ancient written languages prior to the Greek alphabet. Since Greek gave rise to the Latin alphabet that I’m writing in right now, we now use vowels to write. See? 'A…E…I…O…U…and sometimes Y.’ But before Greek, vowels didn’t have much role in ancient written languages, including Egyptian hieroglyphic writing.
Although vowels were not part of ancient written languages, they were a part of the spoken languages.
Try saying 'M-J-K' aloud without vowels. Did it sound like you were mumbling? Now say it with vowels 'M-ah-J-ee-K' separating the consonants. Was it clearer? Could the word be 'magic'? Maybe even a different word?
Vowels are necessary to separate and make consonant sounds distinguishable. So even though vowels weren't written, they were spoken.
A modern example of words written without vowels would be abbreviations. Read these aloud…
Did you say them using vowels? Probably…otherwise no one would understand you. Although these words are written without vowels (and some consonants).
Vowel (and some consonant) sounds are not written but are included when read aloud:
This is how ancient languages, including Egyptian hieroglyphs, treated vowels. Vowels existed in the spoken language but not in the written language.
Since the ancient language spoken in Egypt went extinct a couple thousand years ago we can never know the true pronunciation of the ancient language. Speculation about pronunciation is based on other languages, like Coptic, that are derived from ancient Egyptian.
Some hieroglyphic alphabet characters represent sounds that might pass as vowels by the modern standards, but vowels weren't invented until after ancient Egypt. So for the ancient Egyptians these sounds were weak consonants. These types of sounds might be included when a word began or ended with a softer vowel sound or where it was important to spell things out…like with names.