The Flood Voice Broadcast Alerts App is a Ruby-based Sinatra application that uses the Nexmo Text-To-Speech API to send voice alerts for subscribers who have opted-in to receive flood warnings for their postcode or phone number. The app allows administrators to select who should receive the alert and sends a voice message using the Nexmo API, which includes features such as automatic retry and detection of answering machines. The application uses a virtual phone number provided by Nexmo and stores subscriber information in an SQLite3 database.