Disclosure: MacBook Pro helped me in the computer world since December 2012. I bought it myself around San Francisco and loved its ANSI keyboard (because it is very good to touchtype DVORAK on it).
This year I bought a used MacBook Air from its first owner, again a model bought originally in the U.S. This one is Linux-only since the beginning and it works reliably well.
Here is what I found at the Arch Linux wiki regarding how to mute the startup chime and it worked. Thanks!
#!/bin/sh # # Tested on Linux 5.4.0-56-generic (Ubuntu) on 2020-12-18 # MacBook Air 2014 F=/sys/firmware/efi/efivars F=$F/SystemAudioVolume-7c436110-ab2a-4bbb-a880-fe41995c9f82 chattr -i $F printf "\x07\x00\x00\x00\x00" > $F