We are concerned for your brother's spiritual and emotional well-being. The Bible tells us that our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms (Ephesians 6:12, WEB). It seems your brother is being harassed by an evil spirit, which is using the neighbor's voice to cause fear, weakness, and anger.
Firstly, we must understand that God has not given us a spirit of fear, but of power, love, and self-control (2 Timothy 1:7, WEB). Therefore, your brother can stand firm against these spiritual attacks. He should put on the full armor of God, which includes the belt of truth, the breastplate of righteousness, the shoes of the Good News of peace, the shield of faith, the helmet of salvation, and the sword of the Spirit, which is the word of God (Ephesians 6:13-17, WEB).
In response to his question about praying against the neighbor's voice, Jesus teaches us to love our enemies and pray for those who persecute us (Matthew 5:44, WEB). However, this does not mean we should pray against individuals; rather, we should pray for their salvation and deliverance from any evil influences. Your brother should focus his prayers on his own deliverance and protection, and trust that God will deal with the neighbor according to His will.
Let us pray together for your brother's freedom and peace:
"Heavenly Father, we come before You in the mighty name of Jesus Christ, seeking Your divine intervention in our brother's life. We rebuke the evil spirit that is harassing him, and we command it to leave in Jesus' name. Lord, fill our brother with Your Holy Spirit, giving him strength, courage, and peace. Help him to put on the full armor of God daily, and to stand firm against the enemy's attacks. We also lift up the neighbor to You, Lord, asking that You would have mercy on them and set them free from any evil influences. We trust in Your power and Your will, knowing that in all things, You work for the good of those who love You, who have been called according to Your purpose (Romans 8:28, WEB). In Jesus' name, we pray. Amen."
Lastly, it is crucial for your brother to have a personal relationship with Jesus Christ. If he has not already done so, he should confess with his mouth that Jesus is Lord, and believe in his heart that God raised Him from the dead, for it is with the heart that one believes unto righteousness, and with the mouth that one confesses unto salvation (Romans 10:9-10, WEB). If your brother is already a believer, encourage him to draw near to God, and God will draw near to him (James 4:8, WEB).