This tutorial demonstrates how to stream audio into a call using the Nexmo Voice API, allowing users to play holding music or messages during calls, and even place callers into a conference while streaming audio. The application uses Java with the Spark framework, utilizes the Nexmo client library for authentication and configuration settings, and requires a Vonage API account, virtual phone number, and ngrok for public accessibility. The code guides users through creating a Voice API application, linking their Nexmo number, answering an inbound call, playing audio into the conference, and testing the application. By following this tutorial, developers can create a custom Java application to enhance their Nexmo Voice API experience.