It would be cool if an advanced user could specify a sound file to use as the warning sound. Something like TF2's hit sound customization.

I don't think it would have to be super complicated, maybe a text box where one could enter a Sound ENUM value. More ambitiously, a way to choose a custom .ogg file so I could convert my TF2 hitsounds over.

If it isn't fancy, people will open lots of support tickets because they don't understand how to use it :(

But you can always open the jar in a zip viewer and replace the assets/durabilityviewer/sounds/tool_breaking.ogg file.