Add this shortcut in run shortcut action while creating the automation.It takes shortcuts input as base64 encoded mp3 files.

Wondering how to create an automation:

Shortcuts App -> Automation Tab -> plus icon(top right)-> Create personal automation -> Select Charger at the bottom of the list -> Click on "is Connected/Disconnected"(as you please) -> Next -> Add text(fill the text with base64 encoded mp3) -> Add run shortcut action and select this shortcut -> Next -> Disable ask before running -> Done.

Now you are good to go.

In case you do not know how to convert the sound to base 64 text. Here's the link: Sound<->Base64