When the people saw the thunder and lightning and heard the trumpet and saw the mountain in smoke, they trembled with fear…Moses said to the people, “Do not be afraid. God has come to test you, so that the fear of God will be with you to keep you from sinning.” –Exodus 20:18, 20

Any search on YouTube about “trumpets in the sky” will yield a whole bunch of smartphone recordings about this type of angelic trumpet manifestation, with a variety of interpretations. This video comes closest to the sounds I heard on September 16th: