The Retrieval

Director Chris Eska mentioned (at 0:01:05 in the DVD commentary) that the flashes of distant cannon fire in the opening scene are visual effects that in real-life would precede the sound of the cannon fire by several seconds, but when the correct time delay was used the screen looked strange, so the filmmakers made the flash and the sound appear simultaneously to avoid throwing the audience off from the beginning. The flashes were about 3 seconds apart. Since sound travels about 1 mile in 5 seconds, artillery 3 miles away would have a 15 second disconnect between the flash and the sound.

At the very beginning of the movie, as the boy walks toward the cabin, there is a battle - obviously miles away - occurring over the hill. However, as soon as a flash occurs from a cannon fire there is the sound of the firing - sound travels slower than light, so you should first see the flash and then a time late the report of the discharge.