As a celebrant, my fee includes a battery powered PA system. I am happy for you to play music through my PA during the ceremony as well.
At the rehearsal we will practise how to talk into a mic and where the mic will be placed during your ceremony. I like to do rehearsals, as it makes the client feel more comfortable.
The PA is loud enough for a crown of 250 to hear you, so do not worry. It you amplify your voice, you will definitely be heard during the ceremony.