It it were a free action, could it be spammed repeatedly without consequence?
That seems like a really, really bad idea.
Say, a level 16 Bard. His curse song does 20 points of damage.
At that level, with Extra Music, he can use it 20 times.
This means that in terms of 'FIRE EVERYTHING' damage ( As in, PvP and boss fights ) a level 16 bard can blow out his 15-minutely allotment of Bard Song to do 400 sonic damage to everything in the radius (For comparison, that's around the HP a level 22 Dwarven Barbarian with 20 Con and Toughness would have. A Cleric with Energy Buffer wouldn't have a chance.). And he'd still have all his combat ability from other skills (bards are quite strong here without even taking Song into account). On top of that, any survivor is Cursed.
(Oh, and a level 20 would have 672 damage, if you were wondering)
This is without a save, on everyone in the area. It makes Maximised IGMS look like a ray of frost.
With freely spammable Curse Song, a high level bard could stand in the center of the highest level PC's on the server, set them all hostile, and kill them all with his singing in one round. And that goes against our rules of making characters in the form of real-world celebrities (in this case, David Bowie)
A good meet-in-the-middle would probably be to give it a cooldown. Free action would be fine, if you could only cast it every 10 rounds (Almost any bard making use of Curse Song will have Lingering Music, I'd imagine)
tl:dr: It would cause someone to spurt out a Limerick so meaningful that everyone in earshot utterly loses the will to live, and their heads explode.