iPhone 15 - Change sound output when using an external display ?

Hi 3dfx_,

It sounds like the issue may be with the accessories used. You can test another cable and check out the information below found in the article here: Get Help with Apple Digital AV Adapters for iPhone, iPad, and iPod touch - Apple Support

"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."

You can also test these steps below found in the same article shared.

"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 or iPadOS.

For more help, contact Apple Support, to set up service for your:

• iPhone
• iPad
• iPod touch"

If you contact the manufacturer and they state there are no issues, reach out to Apple Support directly to further assist. Get Support

Take Care!