When a mortar/artillery round is incoming, it does not produce a sound until it impacts.
When placing fire support as Zeus, the sound does occur, but it starts far too early, and ends long before the round impacts, when in reality it continues up until the impact.
Rounds fired manually from arty/mortars do not produce any sound at all.
Zeus: Place a mortar or howitzer fire support module. Listen for the whistling of the incoming round, but note that it stops far before the round impacts.
Manually fired rounds: Manually fire rounds at a position with a mortar. Move to that position (via team switch, splendid camera, etc) and wait for the rounds to hit. Note that they hit and explode without warning, there is no audible warning whatsoever.
Several reference videos:
https://www.youtube.com/watch?v=ca9HXAoV7Zw
https://www.youtube.com/watch?v=m6sV3TElqLw (a JDAM, but still relevant)