After some testing, I have now found the cause/partial solution. By default the com.nicoswd.Carillon.plist file installs in LaunchDaemons. After moving the .plist file to LaunchAgents instead, it now works. Unfortunately, the chime plays after you have entered the password, and the desktop is reached. Not during boot. This is probably because fles in LaunchAgents first will be activated/run after user login. But why it does not work when the file is located in LaunchDaemons I simply have no clue.