flute players

[MeinPanzer]

But the horn would be used to sound the charge or the retreat. The fife was used to give the soldiers the marching rythm, and the "pushing" rythm (for the hoplites that is).

Do you have some specific sources that state how the two instruments were used in ancient combat?

[Klearchos]

Do you have some specific sources that state how the two instruments were used in ancient combat?

No, only what I have seen in pottery designs.In a battle scene, there is often a guy who plays an instrument (we call it "αυλός" in greek, I don't know the proper english word.). This instrument can be single or double and is straped around the players head.
As for the horn, I can't imagine how it can give a marching pace...I think it requires pretty strong lungs to sound a battle horn, and it would be REALLY hard for someone to give the pace with it.
But again: no I don't have specific sources.

[DaCrAzYmOfO]

Aside from flutes and the obvious drums, what other instruments were in use?

I'm sure there werent any others, but if someone knows that there were