That is fantastic!  I'm not too surprised by the stretch that was experienced.  That's got to be a heck of a lot of tension the sling is experiencing.

I did not hear the crack.  It could just be my computer speakers.  Great idea though! 

I liked the idea. I think you need to borrow a drone and put it above the trebuchet when firing. I bet you'll hear it then!!

@Dawesome21 I see you had a mention in one of Todd Cutler's recent videos! Maybe you can borrow one of his drones
https://www.youtube.com/watch?v=Vd2UU-x-Hfg

Or, if you don't mind low quality video/audio, I have one I could send you. It uses a microUSB chip to record video, so you have to kind of guess what it is seeing, but it's better than nothing.

Love the sound the guy ropes make.
You could probably tune those and get an actual tune

From what I see there are several reasons you won't get a whip crack.

1) trebuchets use a vertical release, the release cord stays straight and due to wind resistance and the sheer length of it, the tip is just never going to go supersonic.

2) compared to a human throwing with a hand sling, trebuchets just don't move very fast. You'd need to use a short sling, very light ammo and the maximum weight to even get near the overall speed of a hand sling. But it could well cause the trebuchet to tear itself apart.
The release velocity of a treb is high. But the actual speed of the whole sling is fairly slow.
It's the arm length and sling length that create the release velocity at the pouch.
But because of how it's released and the sheer length of the sling. The tip of the release cord is just never going to hit those supersonic speeds.

With a sidearm throw, you actually create a wave through wrist movement, that ripples down the release cord and causes the tip to snap straight at supersonic speeds.

You just don't get the same wrist snap and cord wave/ripple with a vertical release.

Try it, if you can get a whip crack from a vertical release throw, I'll be seriously impressed.
I've never seen it done or been able to do it.
Sidearm, cracking is an absolute doddle.
Because of the wrist snap that sends a small fast ripple down the cord.

But I reckon experimenting with different length and material guy ropes, you could get a simple tune
Or at least a recognisable chord.

Not sure how you'd make a sidearm throwing treb with a surrogate 'wrist', but until you do, I doubt you'll ever get a whip crack.

I suppose you could make snapping joint instead of a fixed release point. That might work.
But without some similar gimmick, you just won't get a crack.

I suspect if it were possible the Romans would have done it, for added psychological effect.
A huge band followed by the missile would have had an enhanced effect on the enemy.

I don't think there's a single report of a trebuchet ever making a whip crack.

But they didn't have the materials and tools we do.
Make that 'wrist' create that wave/ripple and there's a chance you might get a cracking treb.