This is exactly what I work on (and I mean exactly). Most Egyptian texts are written in a phase of the language called Middle Egyptian. Middle Egyptian was only spoken until the mid second millennium BCE, but the written language continued to be used for formal writing for the remainder of Egyptian pharaonic history. Because most Middle Egyptian texts were written during a state of diglossia (the formal and vernacular languages did not match), it's difficult to say much about how this language was pronounced. By the time we get good information about pronunciation in Coptic, 2000 years passed. Most words do not survive into Coptic. Even when they do, the specific grammatical form is not attested, because the grammar of the language changed as well.

Add to that the fact that many Egyptologists never learn Coptic at all, and the result is that we almost never know how to read a hieroglyphic word aloud. The solution generally adopted is to put e's (phonetic [ɛ]) between the known consonants, and to treat some consonants as though they represented vowels. This is done to deliberately mark the fact that we don't know how it would have been pronounced.

You've probably seen this happening in practice without realizing it. The name "Ra" is known from Coptic as ⲣⲏ ([re]), so his name should always be rendered as "Re", but people pronounce it as "ra" because the hieroglyphic spelling is 𓂋𓂝𓇳 (rꜥ in Egyptian transliteration, [r_ʕ] phonetically). Names like 𓇋𓏠𓈖𓊵𓏏𓊪, "Amun is satisfied", may be rendered as "Amunhotep" from Coptic ⲁⲙⲟⲩⲛ+Ϩⲱⲧⲡ or Imenhetep based on Egyptological convention.

The key thing to recognize there is that "Imenhetep" is deliberately wrong. It couldn't possibly be the word's true ancient pronunciation. If you ever see a piece of pop culture where people speak Egyptian, they've probably misunderstood what Egyptologists are trying to accomplish by pronouncing things this way. The list includes Philip Glass's Akhenaten and Assassin's Creed: Origins.

So this is what we do when we read for the most part, we mispronounce things on purpose to show that we don't know. People who are obsessed with vocalization, like me, tend to use the Coptic pronunciation or the various Coptic-based vocalizations. That's a minority practice to be sure. I always read Demotic with Coptic pronunciation in my head, but I know many highly-competent Demotists who don't know any Coptic whatsoever. It's pretty flexible in practice.

I've been arguing forever that the absence of this crucial component of language learning hamstrings our understanding. Many people disagree. That's more of a personality thing. There are plenty of Egyptologists who are more interested in extracting meaning than they are in revivifying the spoken language of ancient Egypt. For them, being able to read the text seems separate from being able to hear it. I think they're incorrect. I did an experiment a few years ago with a Late Egyptian class where I taught one class Coptic alongside the hieroglyphs. The students who had Coptic vocalizations performed better on vocabulary quizzes, but my sample sizes were too small to be confident in the results (p = 0.54 😕). I'm hoping to try the experiment again on a larger scale.

Sources available at r/AncientEgyptian. See especially the thread on Allen's Middle Egyptian.