I´ve lost sound on videos on my iphone 7 & have tried all of the suggested fixes....except restore factory settings (obviously a last resort that I want to avoid). Anyone any new ideas please? Up to date iOS 14.4.2

Adjust audio and visual settings on iPhone

• Go to Settings > Accessibility > Audio/Visual.
• Adjust any of the following: Mono Audio: Combine the left and right channels into a mono signal played on both channels. Balance: Drag the Left Right Stereo Balance slider.

Adjust audio and visual settings on iPhone - Apple Support

When you're on the phone or listening to songs, movies, or other media, the buttons on the side of iPhone adjust the audio volume.

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Lock the ringer and alert volumes in Settings

1. Go to Settings.
2. Tap Sounds & Haptics (on supported models) or Sounds (on other iPhone models).
3. Turn off Change with Buttons.

Adjust the volume on iPhone - Apple Support

If there is no sound on the video, reset to factory settings will not bring the sound back.

Check the following.

Use the iMovie app and add your video to the app. The following will test if audio exists with your video.

Watch this GIF below where I have shown to detach audio and then you may attach any audio.