/sbin/casper-stop already has (or has since had added) code to handle this situation if the file /run/casper-no-prompt exists.
You can create a simple systemd unit file to always create this:
mint@mint:~$ cat /etc/systemd/system/casper-no-prompt.service
# see /sbin/casper-stop
Squashfs is often used in association with an union FS (overlayfs, aufs) to create immutable live images.
Meaning that squashfs image is mounted readonly, and a tmpfs is mounted readwrite. We then assemble these two layers and mount them as an union on /. These assembly and mounting steps are usually done in initramfs before chrooting to the assembled root ...
ISO hybrid images are crazy combinations of iso9660 format and multiple partition tables to make sure it boots everywhere. This is how the superblock looks:
DEVICE OFFSET TYPE UUID LABEL
sdb 0x8001 iso9660 2020-09-26-10-19-19-00 Debian 10.6.0 amd64 n
sdb 0x1fe dos
sdb 0x200 gpt ...
Try to identify where the file grub.cfg is in the iso image (for me it is at boot/grub/). Remove the following two lines in the file:
play 960 440 1 0 4 440 1
This is telling grub to play a tune of 440 Hz with a rest between it with a set tempo (refer to the grub manual for more detail). This is for booting from UEFI.
Remove the system bell ^G ...