If you're not a fan of the way your iPhone automatically adjusts the volume of music and other audio based on how fast you're moving, you can turn off this feature. Here's how:
1. Open the Settings app on your iPhone.
2. Tap Sounds & Haptics.
3. Toggle off the switch next to Sound Check.
With Sound Check disabled, your iPhone will no longer adjust the volume of music and other audio based on your movement speed. So if you're in a car or on a train going faster than usual, the volume won't increase as it would with this feature enabled.