In the age of Jesus, it’s not like you could dismiss a voice talking from heaven as someone playing tricks on you with a wireless lapel mic and a hidden loudspeaker. That just wasn’t a possibility. So these people had to dismiss the voice from heaven as thunder, even though they heard the voice, and what it said. They dismissed the voice as thunder because they did not want to believe, or be confronted with the truth.
They’ve seen this movie before, and so some of them are thinking, “Great! Jesus is here, he’ll calm the storm and all will be well.”
But Jesus doesn’t calm the storm.
He’s just standing there, telling them not to be afraid, in the midst of all this raging chaos.