Why this isn't a duplicate
This is not a duplicate of Can an Unconscious PC hear you?
The crux of my question is that I didn't read the description of Wail thoroughly enough; once the accepted answer (and a now-deleted comment by Purple Monkey) had pointed out that the description says "that can hear her", I then realised why the unconscious creature would not be affected.
I already understood everything in that other Q&A before posting my question, and if this had been simply closed as duplicate without any further explanation (i.e. the accepted answer and Purple Monkey's comment), then my question would still be unanswered, because that other Q&A does not answer my question at all (at best, only by inference).
In short, does the other question point out to me that those affected by a Banshee's Wail must be able to hear her? No? Then it is not a duplicate and those who have marked it as such are objectively wrong to have done so.
The Actual Question
A Banshee (MM, p. 23) has the Wail action:
Wail (1/Day). The banshee releases a mournful wail, provided that she isn’t in sunlight. This wail has no effect on constructs and undead. All other creatures within 30 feet of her that can hear her must make a DC 13 Constitution saving throw. On a failure, a creature drops to 0 hit points. On a success, a creature takes 10 (3d6) psychic damage.
If someone was already at 0 hit points (for example, having failed their saving throw against a different Banshee's Wail), what would happen if they fail their saving throw against this Banshee's Wail (assuming they still need to make a saving throw)?
- The unconscious creature takes one step towards death, as though they took damage (thus making the saving throw meaningless, since they'd take a step towards death either way);
- The unconscious creature just dies (unlikely, but in case there's something written that would imply this hidden in the rules somewhere);
- Nothing, since this isn't damage, its just an effect (meaning the unconscious creature would be better off failing this save than passing in this case!)
- Something else...