Speaking with a loud voice is not really speaking with authority. The loudness of ones voice is meant to help people to hear and ultimately be able to help understand whatever is spoken.
Jesus spoke with authority when he went about doing good and healing those who were oppressed of the devil. In reference to Jesus, the Bible in Matthew 7:29 says,”For he taught them as one having authority, and not as the scribes” (KJV). What Jesus spoke out had the ability to happen in their season.
Those who believe in Christ also speak with the same authority because it is really Jesus who is speaking through them. One day when Jesus sent his disciples for a donkey so that he could use it to enter the city of Jerusalem according to prophecy, what they had to do and say depended on what he told them. Jesus sent them by the word of his mouth and by the same word prepared them for possible challenges that they would face.
Now, this was a challenge that Jesus made sure the disciples were prepared to face. What happened? Well, when they got to that donkey, someone challenged their action and they responded according to what Jesus told them. The disciples functioned within the authority of Jesus by following what he told them to do. In Mark 11:6 the Bible says,”And they said unto them even as Jesus had commanded: and they let them go” (KJV). Jesus has the authority and through his word we function with the same authority.
You have to believe and thus speak from what you believe, but you also have to speak according to the ability God gives. In 1 Peter 4: 11 the Bible says, “If any man speak, let him speak as the oracles of God; if any man minister, let him do it as of the ability which God giveth: that God in all things may be glorified through Jesus Christ, to whom be praise and dominion for ever and ever. Amen” (KJV). Authority is found in Jesus Christ. When you speak the word of God, that word has the authority to heal, forgive and raise the dead.