I mean, that would depend on how everything interacts with those strings.

Are they everywhere and you just pluck them with your fingers basically in thin air? Then mages could just be shredding air guitars all day, which might be hilarious if thats what you want to go for.

Do the strings vibrate with every sound that is made? Then there could be different ways of doing magic. Instumental, singing, speaking, clapping, etc.

Is it only music? And would natural sounds also be able to cause magical effects? Or is physics even the other way around in your world? Do sounds just occur and the right whisper makes the trees sway and a loud rumble can make the earth quake?

From there you could get into a Tolkien-like area, where the world was just sung into being in the first place.

Just to throw a few questions/ideas out there, where this might go.