Why are the two accounts given in Acts 9.7 and Acts 22.9 not a contradiction?

It could be that they heard the voice of Saul, saying, "who are you?" and "what will you have me to do?" but saw nobody that he spoke to, which surprised them; for it is certain they did not hear the voice of Christ, who spoke to Paul - Acts 22:9 - or if they heard the voice of Christ, it was only the sound of his voice, but did not understand what he said; but the first explanation seems the best way of reconciling the two passages.