You basically want a cord end/release tab that actively slows the cord down.
I don't get cracks with paracord knots, but they make a fairly loud noise that seems to carry
When I was slinging at hodd hill, I 'occasionally' did a few ammo less throws, just to see Pete - 100 metres away - duck and flinch every time
Mind you when I slung the golf balls directly at him, because I was getting a full 90 degree curve out of them, he practically crapped himself
So a release tab that also has a lot of extra air resistance would probably do the job.
As far as quiet ammo goes, spherical ammo doesn't make much noise.
Moulded spheres and a slowed down release cord would probably get it as quiet as possible.
So a compact release tab that unfolds into something with a much larger surface area should work. Or shaped for maximum air resistance. I'm not keen on tabs as you can't easily adjust for different grips.
But you know me and widget challenges
Not sure how it would effect the sling performance though.
Hmm, got a few ideas, I'll have a play tonight while the torus and flying saucer disc moulds are being made