Chabad.org — English translation + Rashi
Torah Temimah: “חצוצרות כסף” — silver is the metal of assembly; on Rosh Hashanah the shofar is plated with gold for the honor of the day, but ordinary gathering instruments are silver.
English: The shofar of fast-days has its mouth plated with silver; why is that of Rosh Hashanah plated with gold? Because every assembly-instrument is of silver, as it is written “make for yourself two silver chatzotzros.”
Metal of assembly — silver marks the instrument used to gather the people.
The Torah’s choice of silver sets the standard for instruments of public call.