Thorns Enchantment

It's not "guardian sound", it's thorns enchantment sound. Guardians just can use something like thorns effect.

When PLAYERS are attacking ONE ANOTHER with thorns on their armor, it plays the guardian sound.

Don't you understand that there isn't such a thing as "guardian sound" in terms of thorns effect?

The sound you're referring to is not necessarily a guardian sound. It's the sound of the "thorns" enchantment. Guardians happen to always have the "Thorns" enchantment, if you will.

The ID of the sound event is enchant.thorns.hit.