You may want to consider sound for an outdoor wedding, although even then it depends on the space. If it’s a small gathering, again, you probably will get more quality sound bang for your buck without it. (And, you won’t have to pay the extra fee to have the musicians get there early to set up the equipment.) However, if it’s a cast of thousands, definitely consider boosting the sound. (You may have already hired sound people for the wedding party members who speak and officiating person in this case, and they can take care of all your needs. Or, you may have hired a band or DJ for later in the evening; you could consult with them about providing sound for the ceremony.)
Now, the reception is a whole other animal, so let’s talk about that. Your musicians have played beautifully for the ceremony, everyone could hear them clearly and are now complimenting you on your excellent taste in music. The musicians have moved to their reception area and have begun to play some whirling waltzes, and……wait a minute! I see them playing, but why can’t I hear them?
Well, if you have a crowd of over one hundred guests, and they are now all clinking their glasses of champagne and catching up with each other, you may have a drowned out string quartet. This is an excellent time to have sound amplification so those tunes can be heard, but not in a way that cuts into conversation. Alternatively or in addition, arrange to have them up on a balcony or riser so the sound can float down.