Djs can provide the ceremony sound for weddings if needed. They can provide the service of setting up the speakers and having the microphones to work correctly. They can also be behind the mixing board to adjust volumes and in case of feedback or if someone needs a boast or reduced in volume based on their voice is soft or too loud. The dj can also cue up the music for the pre-ceremony music and entrance music for everyone to come in. 

Or the couple can do it DIY. Some djs only bring one speaker but I always bring two for the ceremony. It has a good balanced sound to it. A smart phone or tablet can be connected to the speaker via Bluetooth or a cable. You would still need someone to man the music to push play at the appropriate times. If it’s a small gathering then you would not need microphones or if it’s a chapel that has their own microphones then you set. But if you want to diy microphones, its crucial to have someone overseeing the volumes. 

So, which is better between getting a dj to do your sound versus diy? A diy rental would run about 150 to 300 dollars and you would have to pick it up and set it up or you would have it set up for you. The question is who ever is working it are they going to know how to use the gear correctly. A dj will be using their own equipment so they know how to use it. The cost for a dj to do the ceremony can range 300 to 500 dollars.