General Brazilian
amplificar a voz
Most common usage across Brazil
São Paulo
amplificar a voz / reforçar a voz
Both terms are commonly used in formal and informal contexts
Rio de Janeiro
amplificar a voz / dar volume à voz
Colloquial usage may include 'dar volume'
Portugal
amplificar a voz / dar mais alcance à voz
Portuguese Portuguese may emphasize 'reach' aspect more