Speakers and horns are both rated on the maximum wattage at which they can broadcast. The higher the wattage rating, the louder they will sound. CB radios provide only a maximum of 7 watts of power to the external speaker and PA.
Regardless of the wattage rating of your speaker/horn, it won't be louder than 7 watts UNLESS you further amplify the signal. To overcome this bottleneck and generate more volume, you'll need to purchase an amplified PA horn or speaker - or use a device that can power the speaker/horn at more than 7 watts.