All praise to Allah Swt and blessings on Prophet Muhammad peace be upon him His family and Companions
Abu Qatadah narrated that : The Prophet (peace be upon him) said to Abu Bakr: "I passed by you while you were reciting and your voice was low." He said: "I let He who, I was consulting hear." He said: "Raise your voice." Then he said to Umar: "I passed by while you were reciting and your voice was loud." So he said: "I repel drowsiness and keep Ash-Shaitan away." So he said: "Lower your voice."(Tirmidhi)
Based on the above Narration Scholars have mentioned that During Salah one must recite in a way that a person is able to hear his own voice with the movement of lips but not loud enough that others could also listen to him and The Dhikr or supplications can be done in heart.
Allah Swt and His Messenger knows best.