They have various reactions, these could be paired with chatter sounds
roger, wilco, aye aye, (when accepting orders)
contact, contact front!, enemy spotted
taking fire, returning fire,
they can panic, auto-react out of control
"let's rock!" scene from Aliens
this would require a Vasquez - like character (red bandana and all the jazz).
units can elect to fall back by squads and lay down covering fire (with the incinerators), they can also deploy smoke grenades to cover their retreat, pull wounded comrades etc.