Theophylact of Ohrid

Since very many were crude and ignorant, they took the voice for thunder, even though the voice was articulate and very clear. For they quickly forgot the words of the voice, retaining only its echo. Others remembered the very words of the voice: "I have both glorified it and will glorify it again," yet not understanding the meaning of these words, they thought that an angel had spoken to Him, and therefore these words, as spoken by an angel, were incomprehensible to them.

Commentary on John ↗

Ambrose of Milan

How does God speak? Does He have a physical voice? Not at all, but He pours forth oracles with a certain superior power than a physical voice could have. The prophets heard this voice; the faithful hear this voice, but the wicked do not understand. Finally, in the Gospel you have, because the evangelist heard Him saying: "And I have glorified, and will glorify again" (John 12:28); but the Jews did not hear. For they said: "It thundered" (Ibid., 29). …

On Paradise, Chapter 14.69 ↗

Gregory the Dialogist

God speaks in words by an Angel, when nothing is displayed in outward appearance, but the words of the Heavenly saying are heard; as on the Lord saying, "Father, glorify Thy Son, that Thy Son may glorify Thee;" it is immediately replied, "I have glorified, and will glorify Him again." For God, Who speaks without time, by the power of inward impulse, uttered not in time that voice by His own Substance, which voice, circumscribed by time, He made plain by human words. …

Morals on the Book of Job, Book XXVIII, 4 ↗

Thomas Aquinas

Next we see the opinion of the crowd, which was wondering about the voice, The crowd standing by heard it and said. In this crowd, as in every other, some were dull and slow to understand, and others were more perceptive; yet all of them failed to identify the voice. Those who were slow and carnal only heard it as a sound; so they said that it had thundered. …

Commentary on John ↗