Musical instruments. Horns, Tubas, Even Pipe Organs. Drums are used in Movies but those were not used for battle. Here is a great video explaining it. From the History Network no less. No aliens are in the video.

https://thehistorynetwork.org/battlefield-communications/

Flags were used a lot as well in the ancient world. Say you are a commander of a small group of men. You would listen for the horn blasts. Before the battle started your general would have laid out basic plan with a few contingencies. So if you heard the advance call you knew ahead of time where you specifically were to take your men.