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:
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.