How does anyone know what Latin sounded like?

Why past tense?
The Catholic Church uses it today
If you mean how did Caesar use the language no way to know but it was extensively written and preserved.
But I bet how it sounds in the Vatican is pretty close

What Latin Sounded Like—and how we know

One of the first things I learned in Latin class was that we were going to learn the formal ancient pronunciation style, and that it was different from what the Catholic church uses today in that the letter V was pronounced like a W in modern English, and that there were reasons for thinking this, developed by scholars who carefully studied the existing texts from the time. Much of the rest of the pronunciation is pretty similar to formal Italian, which apart from all the regional accents is rather regular. Venice was also not overrun between ancient times and medieval times, providing more continuity of evidence.

That is, linguists and phoneticists are very patient and detail-oriented people who have spent lifetimes deducing and discussing and coming up with understandings that seem to make the most sense, though it’s possible they may be mistaken on some points or details, as clearly there is no actual way to hear an example. However there is quite a lot of contemporary rhyming poetry and some discussion of language, which has been analyzed by such folks.

There is a nice paper/article on the subject which in sum answers your question in section 3, copied below:

“Do we know how the Romans pronounced Latin?
Surprisingly, yes. The details of the reconstruction are given in W. Sidney Allen, Vox Latina (written in English), Cambridge, 1965. There are several main sources of knowledge:

* The Latin alphabet was meant to be entirely phonetic. Unlike us, the ancient Romans did not inherit their spellings from any earlier language. What you see is what you get.

* Language teaching was big business in Roman times, and ancient Roman grammarians give us surprisingly detailed information about the sounds of the language.

* Languages derived from Latin give us a lot of evidence. In fact, many of the letters of the alphabet are pronounced the same way in French, Spanish, Portuguese, and Italian. It stands to reason that the original Latin pronunciation has survived.

* Spelling errors made by the ancient Romans are very informative. If two letters are often mixed up, they must sound fairly similar. Likewise, if two letters are never mixed up, we know they sounded different. [example snipped by Zaku]

* Finally, transcriptions into other writing systems, such as Greek and Sanskrit, often pin down the ancient pronunciation of Latin very precisely.”

Wow. Seriously, WOW

Yeah, we do. Latin is a dead language only in that no one uses it for everyday speech anymore, but it is, and always has been, very well documented and it heavily informs many languages being spoken today. Even English, a Germanic language, takes a fair amount from Latin.

I recently read about people determining how English was pronounced when Shakespeare’s plays were new. Poetry provides a lot of clues, because they can see words that rhymed at the the time which might not today.

Here’s a 10 minute video on it:
Shakespeare: Original pronunciation

The “clasical” pronunciation of Latin makes sense linguistically. The “Ecclesiastical” version, heard in the Catholic Church, actually came about after the various “Vulgar” dialects had already morphed into the Romance languages of modernity, and is pronounced more like Italian.

By reconstruction.

Plus, there are grammar books on Latin, written back in ancient Rome and Middle Ages (for both Classical and Medieval Latin, respectfully).

Thanks so much, great answers!