for me getting the disk percentage, i do df -h | awk '$1 == "/dev/nvme0n1p5" {print $5}'

my main disk is just nvme0n1p5 so this works. So you could use some sort of audio program to notify whenever disk usage hits let's say > 50%.

Sadly i dont know any specific audio programs that can do this, but i do know about ffplay, you need to download an audio file and then you can play it with

ffplay audio.mp3