There are numerous synonyms for the phrase "sound horn." Some common ones include beep, honk, blare, toot, and hoot. Depending on the state or country you are driving in, different terms may be used. In some areas, the phrase "blast the horn" is used instead of "sound horn." It's important to note that while using your horn can be necessary for safety reasons, it should be used sparingly and only when absolutely necessary. Overuse of the horn can be annoying and disruptive to those around you, so it's important to be mindful of your surroundings when using it.