Hello clavissiere,
Thank you for reaching out in Apple Support Communities. With the HDMI TV adapter, is it manufactured by a third party company or are you using the Apple branded Lightning Digital AV Adapter? About the Apple USB-C to USB Adapter
If you're using one of the Apple branded adapters, we suggest to follow these steps as it can help resolve your issue:
Get help
Find out what to do when you have the following issues.
If you see video but can't hear audio
Make sure that you're using a Digital AV adapter (which has an HDMI port). If you're using a VGA adapter, it doesn't carry audio signals. To hear audio with a VGA adapter, you need to connect to the headset jack on your TV, monitor, projector, or stereo.
If an app doesn't display video or play audio
The app might not be compatible with your adapter. Check compatibility on the app's info page in the App Store or contact the developer.
If an alert says that an HDCP-compatible device is required
Your content might not be compatible with your adapter. Movies, TV shows, and certain streaming video apps require High-Bandwidth Digital Content Protection (HDCP). If you try to play these movies, shows, and streaming videos using a non-HDCP digital connection—like the Apple VGA Adapter—an alert will ask for an HDCP-compatible device.
If you need help with other issues
1.Disconnect and reconnect your adapter from your iPhone, iPad, or iPod touch and from your TV, monitor, or projector.
2.Make sure that your VGA or HDMI cable works, since the issue could be with the cable.
3.Remove any VGA or HDMI extension cables or converters. Accessories that convert a VGA or HDMI signals to different video formats (DVI, Composite, Component) aren't compatible.
4.Make sure that you have the latest version of iOS.
For more help, contact Apple Support, to set up service for your:
iPhone
iPad
iPod touch
This information is outlined in this helpful article: About Apple Digital AV Adapters for iPhone, iPad, and iPod touch - Apple Support
We look forward to hearing back from you with an update.
Kind regards.