Because of the limitation of Android system, it is impossible to transmit the sound to TV or computer when mirror screen. Most screen mirroring Apps on the market do like this. But if the system of your phone is Android 10+ and your TV box is Android 6+, this problem will not happen.
Recommendation: If you need audio, please connect phone to wired earphones or Bluetooth speaker to get better experience.
If you just want mirror/cast/stream a local video on your phone, it is recommended to select feature [Mirror Media], which has sound.