I can answer both questions. Firstly the ch-ch sound is present in XWA, once you release the trigger and your last bolt is clear you'll hear it.
As to the refire problem, its been present since X-Wing, we can't do anything about it and its basically a bug. There is one value which is applied to the refire rate of all guns which is why they are all the same, that value is, more or less, correct to the movies. However the formular used to calculate the refire of linked guns is wrong
Where r= the generic refire value applied to all guns g= the number of gun sets firing, i.e. the number of salvos per cycle and d= the resulting delay between salvos the formular should be this:
Instead g actually = the number of guns firing and the formular is:
The result is that instead of the delay between every full cycle being the same the delay between every shot is the same, so the more guns linked the slower they fire.
Does that answer your question, its a little complicated but basically some guy 20 years ago got his maths wrong and thats why the guns all fire slow.