They said "loose" or "shoot". Fire is an anachronism, as it applies to firearms, and would not have been used to command archers.

According to the Taktika of Leo VI the Wise, a Byzantine military manual from the 10th Century, the command for archers to fire was "τοξεύειν!", probably transliterated as "toxeyein!", which roughly translates to "shoot the bows!".

during a battle what did people say when they wanted to release a volley of arrows?

In which country? In which period? In which language?