Some basic musical theory might help give you some ideas. High notes vs low notes. Major vs minor keys. Fast vs slow tempo. Harmonies. All things that could correspond to how magic changes reality. Music gets complex, and that complexity can reflect the different ways that magic manifests.

You also might consider how people are making these frequencies. Possibly there are certain materials required to make the sounds. You could have musical instruments or sound-making devices that have a large range but are difficult to learn, simpler devices that can be played more easily but with fewer efffects or less nuance. Maybe single-sound devices that only generate a single effect, meant for novices, or as a safety measure. Perhaps some effects require multiple people making sounds together, either to add complexity (like an orchestra) or for pure volume (like a marching band).

Some groups might be more technical about their use of magic, only using the minimum complexity required to create an effect, while others may treat it as an art that is meant to be aesthetically beautiful as well as functional.

Lots to play with.

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.

I think there would be different levels of "music" like simple patterns of tones, then melodies, polyphony, symphony, etc. Various schools of magic as many as there are musical traditions and scales or the equivalent in each tradition. Perhaps even culture develops to match with this new type of "music" as a type of ritual performance, like sacred dances and ritual musical dramas.

There is a series called the Book of the Stars I believe, they had an interesting magic system. The constellations of the world formed letters which formed an alphabet, and you spell words with them. There were alternative forms of magic too where one weaves together many syllables into a long droning recitation or something, I forget exactly. But that is kind of what I'm envisioning with the "levels" building upon each other