If the Rosetta Stone taught us how to translate the language of Ancient Egypt (hieroglyphics, I'm pretty sure), it wouldn't teach us how to pronounce names like Tutenkhamen or Nefertiti.
hi! not discouraging any of the subject experts from jumping in here, but fyi, you may be interested in these
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. ...
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