This is a matter of sequence. When you start mirroring when your iPhone or iPad is on the home screen, there is no audio. When you start the mirroring when you are IN the app that you want to mirror, there will be sound.
Using Safari Browser:
Please update your iPhone or iPad to iOS 13 to have sound from your Safari browser during mirroring. Also, do not forget to disable to mute button 🔕 which is positioned on the side of your device. Turn down the sound of your phone/tablet and up on your TV.
At this time, Safari, Chrome and FireFox browser won’t allow sound during mirroring with iOS 11 or 12. We recommend to download ‘Dolphin Browser’ for free to enable sound for your browser. Download from the App Store: