Thanks for visiting the Apple Support Communities, rxyy,
To get started investigating your audio while wearing headphones, we would recommend seeing if the issue persists when using another app, like Music, to listen to audio. If so, we would also recommend seeing if the issue persist when using a different set of head or earphones.
If the issue continues with multiple source apps and multiple headsets, we would recommend next verifying your iPhone volume settings with Adjust the volume on iPhone - Apple Support and Change iPhone sounds and vibrations - Apple Support, as well as the headphone audio level settings: Use headphone audio level features on iPhone - Apple Support. Any of the settings here may impact the way the iPhone controls your audio.
We can also suggest checking app-specific audio settings for YouTube, and other apps you're using. For example, if you turn off in-game sounds, does the YouTube audio still change when going into the app?
Don't hesitate to respond back if you still need help.
Best!