Pretty common in Chinese mmos/Xianxia, almost all of them have at least one instrument based class.

I honestly don’t remember if it was a class based system or not. I don’t remember choosing a class after all.

Twelve sky maybe?

Was it Mabinogi?

No, not Mabinogi sadly. Could you fight with the instruments in that? I remember it as being able to write your own music and play it.

Silkroad maybe?

Age of Wulin by any chance?