The trumpet, or similar instruments, are found in a number of stories in the bible and other ancient religious writing so it should be obvious that it has symbolic meaning.  In general, a trumpet symbolizes the presence of God.  Angels trumpeted when Jesus was born.  Trumpets brought down the walls of Jericho.  
    They can also be a call to do something or to indicate that something important is about to happen.  One of the signs of the end times and the return of Christ is trumpets blowing (Matt 24:30).  A trumpet is blown six times in revelation, each time announcing something different, than the final seventh trumpet to announce the return of Christ.  The Jewish holiday of Rosh Hashana is also called "the Feast of Trumpets" and is the Jewish New Year.  Perhaps this is where we got the idea of blowing horns on New Year's Eve.
    Trumpets have been used for thousands of years to announce the presence of a king or other great person, to announce a march into battle, or to announce retreat.  The giant Swiss trumpets called alpenhorns were used to send messages.  On New Year's Eve, loud noise is made with noise makers, horns, fireworks and shouting.  This is intended to scare away evil spirits.
    In dream symbology, if you hear a trumpet in a dream you will receive a surprise revelation, but if you are blowing the trumpet, you have ego problems.