How can the Quran be the Word of God when it is composed of created letters that we pronounce, and of the created sound with which God spoke to Moses (peace be upon him)?

The speech of the created is created, and the speech of the Creator is uncreated. If a created being delivers the speech of the Creator, then his voice and manner of delivery are created, but the heard speech remains the uncreated word of Allah.

To clarify: Speech is an attribute of the speaker. No matter how much others hear it, memorize it, write it, or utter it, the attribute does not transfer to or reside in others. For example, the Hadith "Indeed, deeds are by intentions" is the speech of the Messenger of Allah, peace and blessings be upon him. Its presence in your heart, your writing of it, or your uttering it does not mean that the attribute of speech, by which the Prophet, peace and blessings be upon him, was characterized, has resided in you. Therefore, speech is attributed to its original speaker, not to one who merely conveys or delivers it.