When Jesus returns, this will be the sound that will be supernaturally heard by every ear on earth. Then, the ones who have already accepted the Messiah in their hearts will ascend into heaven leaving this earth. The ones who are left behind will then know for sure that Jesus was Lord. They will have another chance to come to God, but it will come at a MUCH higher cost. Think of the Holocaust. It will be FAR worse.
“Rest in the Lord, and wait patiently for Him; fret not thyself because of him who prosperth in his way, because of the man who bringeth wicked devices to pass.” Psalms 37:7