I would like to suggest a warning sound (and perhaps some textual feedback as well) when a ship's armor drops below a certain percentage (maybe 30% or 25%).

Not a practical necessity in terms of gameplay but a psychological tool to provide an "Oh, crap!" moment to combat; it provides a moment of decision during which the player has to make a choice (flee or fight). Think of Halo and that moment when your shields drop or and you'll get the feeling I'm thinking of.

RegisterEvent(function()
local d1,d2,d3,d4,d5,d6,dmg,dmgmax=GetActiveShipHealth()
local health = dmg/dmgmax
if health < 0.3 then
gksound.GKPlaySound('lockon', 1)
print('Low health!!!!')
end
end, 'SHIP_UPDATED')

like that?

But as opposed to Halo, Call of Duty, Gears of War, to name a few, VO has no cover to speak of, so the warning would be essentially pointless or even dangerous because it distracts from the fight at hand.

Also, you should know, in a general sense, how much armor you have left, without looking at the counter.

Or have a counter where you can easily see it, still, it's personal preference.