i know for a fact that sound does have a delay in this game but only for the first sound to occur.

for example: i fired 3 mortars at a target 800m away and fair enough i saw a huge dust cloud first and heard the sound about 2.8 seconds later but when the next 2 hit the sound occurred exactly when the mortar hit.

if you wait say 5-10 seconds then do it again the same thing will happen