Latin followed the alphabetic principle with few exceptions (U and J were later invented to fix the main exceptions). We know almost exactly how it was pronunced because of that and remarks contemporaries recorded on how speech sounded. For example, R was called the littera canina because of the trill. It is especially noticeable when you have two of them together like in terra.
The main thing we do not know is how regnum was pronounced. We know it was either of two options for interpreting gn and a third choice is that both were acceptable. People are also unsure how 4 of the short vowels sounded. Some say that they have slightly different sounds while others say that they are just short versions of the long vowels like the other two (A and Y). It is possible both variations co-existed.
We also know that western Romans often mispronounced Y as I since they had trouble rounding their lips for Y. Y had been introduced for transliteration of Greek loanwords, so it was not a native sound for the western Romans.
The main thing we do not know is how regnum was pronounced. We know it was either of two options for interpreting gn and a third choice is that both were acceptable. People are also unsure how 4 of the short vowels sounded. Some say that they have slightly different sounds while others say that they are just short versions of the long vowels like the other two (A and Y). It is possible both variations co-existed.
We also know that western Romans often mispronounced Y as I since they had trouble rounding their lips for Y. Y had been introduced for transliteration of Greek loanwords, so it was not a native sound for the western Romans.