By default, stable volume is turned on when you're watching videos on YouTube. Stable volume balances the range between quiet and loud, continuously adjusting volume levels to reduce variations in sound.
Note: This setting is only available on the YouTube mobile app. Stable volume isn't available for all videos. Music is not affected by this feature.
If you don't want stable volume turned on, you can turn it off in your settings:
- Tap your profile picture
.
- Tap Settings
.
- Tap General.
- Turn stable volume on or off.
. 
. You can also turn it off while watching a video:
- At the top of the video, tap settings
- Tap Additional settings.
- Turn stable volume on or off.