John Chrysostom

And they which were with him heard the voice of Paul, but saw no person to whom he answered-for the Lord suffered them to be hearers of what was less important. Had they heard the other Voice, they would not have believed; but perceiving Paul answering some person, they marvelled.

Homily on Acts 19 ↗

Theophylact of Ohrid

"And the men who journeyed with him stood speechless, hearing a voice but seeing no one." The light shines upon all who were with him, so that they might bear witness to this apparition, but it blinds not all of them, but only Paul alone, so that they would not think this was some common and seemingly accidental misfortune, but so that it would be revealed that this was entirely the work of Divine Providence. Why then did those who were with him not believe? So that they could be trustworthy witnesses of what happened to Paul. …

Commentary on Acts ↗

Oecumenius

Those who were with Paul did not hear a voice from above, but Paul's voice. And they saw no one to whom he answered; Paul alone saw the one from above. For when they led him in, Paul himself says, "Those who were with me saw the light and became afraid, but they did not hear the voice of the one speaking to me." (Acts 22:9) For if they had heard that voice, they would not have believed. Seeing Paul answer, they were amazed. And this did not happen at first so that it might be shown truly that Christ rose. …

Commentary on Acts ↗