It's likely you are trying to mirror using AirServer Connect, an app on the Google Play Store. This application is a network helper tool that helps you find AirServer on more complex networks using AirPlay. Due to limitations with Google Play Store apps, it's not allowed to transmit audio from the device using this application but you can use the native Google Cast functionality, or Miracast on some Samsung phones. However, AirServer Connect is not, or should not be, necessary within smaller or home networks.
If you are unable to locate AirServer without it, there might be something else going on, for which we could refer you to one of the articles listed below: