Use of whistles to relay commands in battle

[kazeryu]

...so I suppose for the moment we're going to have to accept that what was seen in "Rome" was conjecture in the flavor of 'modern' military signaling.

Conjecture, yes, "modern", not so much - while the exact authorship and dating of The Art of War is debatable, it appears that everyone agrees it was written no later than 300 BC.

7.17: The Book of Military Administration says: "As the voice cannot be heard in battle, drums and bells are used. As troops cannot see each other clearly in battle, flags and banners are used." (Samuel B. Griffith's translation)

I'm sure there are plently of other ancient historical precendents for sound and flags being used in military operations as signals, but that was the easiest one for me to quote.

Once you have a certain number of men and a desire to do anything more complicated than general melee, you need some form of communication -- it might be whistles, bells, horns, drums, etc., but it will still have to exist! (Sending messengers around becomes impractical after a while...)