While playing, I sometimes find that I don't look at the grey text that comes up saying metal extractor under attack, or some other variant of that. The only unit that actually plays a sound when attacked (that I know of) is the commander.
Could it be implemented that a red X drops onto the mini-map when units are under fire, about every 30 seconds or so, unless the player is looking at the battle and that a sound similar, but different to the 'Commander Under Fire' sound is played to go along with the red X dropping onto the mini-map?