People have been teaching each other how to pronouncing Latin non-stop since Ancient Rome. So, while it's probable that there's some deviation, it's not a guess at all.

Besides, most Latin languages agree on most of their sounds, so even if we didn't know how it sounds, that guess would be very highly educated.

Latin is actually very well attested, both in the classical and ecclesiastical pronunciation. There’s lots of languages, like Egyptian, where the phonology is pretty speculative, but Latin isn’t one of them.

Linguists have analyzed ancient texts (through common misspellings, poetry, etc) and mostly pieced together what it sounded at like at certain times/places. Pronunciation is a moving target and not much worth getting hung up on.