So we can’t know exact sound but we can be reasonably certain for a variety of reasons. While this is a huge subject, see this reply I gave to an earlier similar question for a starting point.

Regarding things like tunings, that is actually an interesting subject because we are likely not playing with the exact same tuning, particularly prior to the 1750s.

These days, nearly everyone in Western music uses something called equal temperament for tuning instruments. Equal temperament means that we divide an octave into equal parts. This became standard practice by the mid-to-late 1700s.

But before around 1750 there were a variety of tuning styles, including meantone, which focused on tuning the 3rds and 6ths to make for more sonorous triads, and “just intonation”, which focused on a more accurate tuning from one note to the next, but since many instruments can’t be easily changed on the fly, meant that different keys ended up having different moods associated with them.

I can tune a piano to just intonation based on the key of C, but then the relationships between notes won’t be the same if I’m playing a piece in the key of Eb, for example.

And this gets complex! There is debate about which tuning Bach intended in his famous piano book “The Well-Tempered Clavier.” Some say the title refers to equal temperament. Others say “well-tempered” had a specific meaning in his time and was not a reference to that, and the pieces were intended for just intonation. I don’t have an opinion here.

Finally, another significant factor in tuning is what they tuned to. Today, we tend to use the note A sounding at 440hz as a tuning note that we base the others on. That standard, however, is very recent. Tuning pitches varied wildly throughout Europe, with ranges from the high 300s for A into the mid-500s. This is a meaningful difference!

Even as countries started to propose standards, there were differences. The French preferred A435, Germans A440, and so on. A440 wasn’t formally standardized until the mid-20th century when the ISO codified it.

So while we know a good deal about how a piece was intended to be originally performed, even the most perfectly replicated performance may still be tuned just a little differently from how the composer intended for pieces before the A440 standard.

That said, as I noted in the comment I linked above, I don’t think there has ever been a truly “perfect” performance of any composer’s music. This is something written and then sent out for artistic interpretation. I would not want to perform something explicitly and exactly the way someone else did, nor should I! I’ll use again the stage play analogy. A musical score is much more like a director interpreting the script to a play with all the stage directions, sets, and whatnot.