Apple - Turning Power Chime Off

The solution is to set ChimeOnNoHardware to true instead of setting ChimeOnAllHardware to false (note, No instead of All):

defaults write com.apple.PowerChime ChimeOnNoHardware -bool true
killall PowerChime

Here's another solution since the others provided don't seem to be working for you. You can create an empty AIFF file to replace the default chime. The chime audio is located here:

/System/Library/CoreServices/PowerChime.app/Contents/Resources/connect_power.aif

I created an empty AIFF file using Adobe Audition. With SIP disabled, I backed up the original chime and replaced it with my new audio file:

sudo mv /System/Library/CoreServices/PowerChime.app/Contents/Resources/connect_power.aif /System/Library/CoreServices/PowerChime.app/Contents/Resources/connect_power.aif_bak
sudo mv ~/Desktop/null.aif /System/Library/CoreServices/PowerChime.app/Contents/Resources/connect_power.aif

Be sure to fix the permissions on the file:

sudo chown root:wheel /System/Library/CoreServices/PowerChime.app/Contents/Resources/connect_power.aif
sudo chmod 644 /System/Library/CoreServices/PowerChime.app/Contents/Resources/connect_power.aif

No more audio when connecting your power source.

I'm using this tool on all MacBooks after 2015, it allows to quickly switch PowerChime sound

https://github.com/DaFuqtor/powerchime