I have experienced the same, and I vehemently believe it may somehow be related to the sound channels or something similar to that setting.
What happens is that when there are a lot of sources of noise in a battle, many simply get cut or does not play. If there are 150 horses galloping, you can give orders and there won't be any sound. Or you can hit an enemy and it will produce no noise.
It is as if there is a maximum amount of sound that can be played at any given moment, and once the limit is reached new actions just do not produce any sound until there's "room" for more. It is very bizarre, but I've played several large battles with my weapons hitting completely soundlessly among the din of battle on all sides. All of which is played perfectly normally.