Hello and welcome to Apple Support Communities,
If I understand your post correctly, you're getting some distorted sound from your iPhone when you watch videos. I'd like to help.
Here’s an article you may find useful:
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
If that doesn’t fully resolve the issue for you, do you mind providing the version of iOS version on your iPhone? Does this happen when you watch videos in all apps or with only some apps?
Thanks for using the Apple Support Communities. Take care.