system
Given the following post, respond as if you're commenting on Reddit:
human
How do linguists discover the phonetics of ancient symbols?
gpt
Typically via comparison with known languages. Bilingual (or trilingual) inscriptions like the Rosetta stone provide a basis for sound when words like names are transliterated. Monolingual inscriptions in a different script can be helpful, like an Egyptian name written in Greek. You can also find words in surviving languages: Coptic is a surviving language related to that of Egyptian hieroglyphs, and even some unique words in Egyptian Arabic are found to be related to the ancient language. Clues can sometimes be found based on context inside a text, like rhyme or onomatopoeia or puns or confused homophones or whatever else a careful reader may find.

The examples above are all about Egyptian, as it's the most well-known "rediscovered" script, but others will have gone through the same process, being more or less difficult depending on how related those languages are to existing ones and how much written material survives.

Reconstruction of the sound of an ancient language, of course, is far from precise. Language is always changing, and especially in the pre-modern world varied from place to place. Saying something was pronounced as such can only be sort of an idealized approximation at best.