It didn't crackle like that before, I've made many a chicken pit in my life and I've never heard the kinds of sounds coming from that place. It could be the footsteps, but it would be something wrong with the footstep audio then, not just the sound of a hundred footsteps
It's definitely some kind of audio bug, I've heard hundreds of footsteps from other mobs and it doesn't sound like this. Hopefully it'll get patched in 1.13.x
I checked some other versions and it appears to happen in at least 1.12, 1.13, and 1.14, my mistake. I feel like this is still silly though, the beaches look weird and unnatural so I would still consider it a bug. Would love to see it fixed in full 1.14 but considering it is not a new or gamebreaking bug it's understandably probably low priority.