The Rosetta Stone is written in three scripts: Ancient Greek, Demotic and Hieroglyphic. Egyptologists have a relatively good understanding of how spoken Late Egyptian sounded, partly because they have a good understanding of how Early Demotic script sounded, since Demotic is a hieratic script. Late Egyptian was the last "version" of the language that was written using hieroglyphs. Later "versions" used Demotic script for roughly 1000 years (from 700 BCE onwards) which was then followed by Coptic script, which is very well understood.

I say "Early Demotic" because during this era the spoken language changed quite a lot, and Demotic script became more and more disconnected to spoken language, much in the way English words like "eight" and "light" are still written with a "gh" because they come from Germanic words where this sound is pronounced: "acht" and "licht". So Late Demotic is roughly as similar to Early Demotic as Italian is to Latin, and Early Demotic is as close as we get to knowing what Late Egyptian sounded like.

Are you referring to Egyptian Hieroglyphics?