WWII: muzzle velocity (shell speed) of selected tanks and artillery 1939-1945
It should be noted that speeds were not constant, as projectile speeds decrease over time due to air resistance and environmental factors. However, as sound travels at roughly 340 meters per second (depending on weather conditions), shells from the majority of the weapons listed would reach their target before those in the area would hear the cannon fire or the projectile whistle through the air - usually, only incoming mortar fire would be audible to those in the area of impact, due to the arc of the projectile.