What is the evidence that the speech of Allah is with letters and sound?

Question:
May Allah reward you, Honorable Shaykh. This questioner says: We know from the belief of the people of the Sunnah that the speech of Allah is with letters and sound, so what is the evidence that it is with letters and sound?

Answer:
Because it was heard, because it is heard, because it is speech that is heard. And He called out to him, and We called him, and the call must necessarily be with letters and sound. Speech is composed of letters; from what is speech composed? It is composed only of letters. The call, from what is the call composed? It must be from sound; if there were no sound, it would not be heard and would not be called a call. The whisper, We brought it near, a whisper; if there were no sound, Musa (peace be upon him) would not have heard it. So it is necessary that a report come stating that the speech of Allah is with letters and sound. This is derived from the evidence, derived from the evidence. Or is it necessary that a report come stating that the speech of Allah is with letters and sound? Yes.