A. Matter is the thing in itself without its form. That is exactly an Idea, no? The Idea of the horse is horseness without the form of the horse, only the idea of a horse.
B. Then every form is non-essential! Because everything has its matter and its form, and if, as you say, the very fact that it can fail to appear means it is not essential, then it follows that form is always non-essential, and if so your distinction between essential form and accidental form does not exist.

A. The Idea is the form without the matter. A floating form.
B. I didn’t understand. There are parts of the form that cannot be detached from the thing itself, and those are the essential part. A person’s breathing is part of his essential form. So is his intellect.

Regarding light or color or sound, for example: there is a wave that meets my ear and as a result I hear a sound, but in the world itself there is no sound of course, only the wave. And similarly regarding light.

Now, what are the matter and the form of sound or of light? As I understood it, the matter (in the philosophical sense, not in the material sense) is the thing itself that has this wave, and the form is its characteristics, such as wavelength.
Right? Or is the form the pitch of the sound and its decibel level?

Maybe phrased differently: does the form of an object exist even if I am not here? Does it exist in the world itself, or only because I am here does it have a form? Seemingly the horse has a form (height, strength, speed) even if I am not here; that makes no difference.
So I didn’t understand when elsewhere you write that the form is the object as it appears to our eyes.

That is a definitional question. You can speak about the objective form, and that is the wavelength and the other objective characteristics, and you can speak about the subjective-cognitive form, and that is the color and the sound.