The Gemara states that a shofar which is plated with gold at the spot where the mouth rests is invalid. Some Rishonim explain that this is because the sound of the blast would be heard not just from the shofar, but from the gold plating as well. Others say that since the shofar must be placed on the blower’s mouth, a gold plate is not allowed to block that connection. According to this view, it is also invalid to blow into the shofar from a distance. If the shofar was plated outside of the area where the mouth rests, it would only be invalid if the gold changed the sound of the shofar.
[שו"ע תקפו, טז ו־יט, משנ"ב ע, עא, עה ו־עו, ושעה"צ קיב; ביאורים ומוספים דרשו, 69 ו־74]