The Golden Trumpet was supposedly an artifact of immense power; but in fact it was a red herring. The item does not exist in the Ultima game.
The short-lived German computer game magazine Joystick from the early 90's produced an encyclopedic hint section for Ultima IV in its first issues. Among the articles was an entry for the "Golden Trumpet", allegedly a weapon of some sort. It was claimed that its secret location could be learned at the Castle of Lord British. However, in truth neither the item nor any information pertaining to it can be found in the castle, or anywhere in the game. An imprisoned Reaper on the castle's second floor makes vague insinuations towards an item of great power, but it in fact refers to the Skull of Mondain, an object that actually is really powerful.
This item might have been confused with, or based on, the Silver Horn, due to its similarity in name.