By default, the destroyer cannons use the same sound entry as S pulse laser which sort of explains why they aren't very satisfying to use.

https://www.youtube.com/watch?v=Q9PUZ_XnJYY
As with everything else in 3.0, it should probably be considered work in progress and possibly completely unrelated to current destroyer cannons. I do hope it's for the destroyer cannons though as good sound design is extremely important to the feel of combat. I was originally going to make a feedback/discussion thread on the topic of sound (hence the mod I made) as I feel it's a big reason behind a lot of the critique people level at the combat, but as weapons seem to be getting a serious second pass for 3.0 I'll hold off until that has been released.