A.T. Robertson Commentary John 12:29

"The multitude therefore, that stood by, and heard it, said that it had thundered: others said, An angel hath spoken to him." — John 12:29 (ASV)

That it had thundered (βροντην γεγονενα). Perfect active infinitive of γινομα in indirect discourse after ελεγεν and the accusative of general reference (βροντην, thunder, as in Mr 3:17), "that thunder came to pass." So the crowd "standing by" (εστως, second perfect active participle of ιστημ), but Jesus understood his Father's voice.

An angel hath spoken to him (Αγγελος αυτω λελαληκεν). Perfect active indicative of λαλεω. So, when Jesus spoke to Saul on the way to Damascus, those with Saul heard the voice, but did not understand (Acts 9:7; Acts 22:9).