“And those who were with me beheld indeed the light, but they did not hear the voice of him who spoke to me.”
Those who were with him had beheld the light. It was not just something internal. They had heard noises (as we are told elsewhere), but they had not understood exactly what was being said. They had not ‘understood the voice'. Compare John 12:28-29. They were like Paul's listeners, unable to discern, seeing a light, hearing noises, but unresponding.