hi! not discouraging any of the subject experts from jumping in here, but fyi, you may be interested in these

• Why/how was the knowledge of how to read Egyptian hieroglyphs lost? - particularly this bit from /u/CommodoreCoCo

The language itself was not lost. The Egyptian language, in its various stages, was regularly spoken from ~3000 BC until the 1600s AD. Hieroglyphs, as you correctly point out, did fall out of use though. ...

• Exactly how influential was the Rosetta stone in deciphering Egyptian hieroglyphs? - /u/farquier adds

Champollion (the decipherer of the Stone) was very much reliant on previous deductions he and others had made about hieroglyphics and his own knowledge of Near Eastern languages. Champollion had a truly staggering command of Near Eastern languages including most importantly the modern descendant of Egyptian, Coptic. Because of this he was able to deduce that the Ancient Egyptian language was most likely an ancestor of Coptic and would follow similar grammatical rules, have substantially related vocabulary, and so on

• If we had never discovered the Rosetta stone, would we still have deciphered Egyptian hieroglyphs? - /u/farquier gives a more general overview on deciphering

• Without the Rosetta Stone could modern cryptographers decipher hieroglyphics? - more from /u/farquier on understanding of Coptic

• a post from /r/Linguistics Ancient Egyptian - quick overview of Coptic in relation to Middle & Ancient Egyptian

also, there have been loads of posts asking about pronunciation of ancient languages like Ancient Egyptian; if you want to dive deeper, check these out

• How do we know Assyrian pronunciations? (Or any other ancient languages) - contains a pile o' links to more on ancient languages in general, plus several specific languages

• plus this more recent Ancient Egyptian Pronunciations