What is the proof that Allah spoke the Qur’an with letters and sound, and what is the proof that Jibreel (peace be upon him) heard it from Him?

The Holy Quran is the uncreated word of Allah. Allah spoke it in reality, with sound and letters, and made Gabriel hear it. Then Gabriel brought it down to Muhammad, peace be upon him. This is what the Quran and Sunnah indicate, and it is the doctrine of all the Salaf. Allah Almighty says: "And if any one of the polytheists seeks your protection, then grant him protection so that he may hear the words of Allah" (Quran 9:6). And Allah Almighty says: "And indeed, it is the revelation of the Lord of the worlds" (Quran 26:192). And He says: "Say, 'The Pure Spirit has brought it down from your Lord in truth'" (Quran 16:102).

Furthermore, the hadiths confirmed that Gabriel heard the Quran from Allah Almighty and conveyed it to Prophet Muhammad, and Prophet Muhammad conveyed it to his Ummah.