Nexmo's Selective Audio Controls allow supervisors to listen to calls without being heard by customers or agents. This feature uses participant IDs and conversation actions to control audio flow. A Node.js application is created using Express, which serves an answer URL that returns a valid NCCO based on the caller's role. The application generates three types of NCCOs: customer, agent, and supervisor, each with specific parameters to control who can speak and hear in the conversation. Once the application is complete, it uses a findParticipants method to search for leg UUIDs by role and stores them when a request is made to the answer URL.