Turn off Bluetooth on your iPhone 8 to ensure that your music is not being played through a different device, such as Bluetooth speakers or headphones. The audio is most likely being played through the Bluetooth device connected to your iPhone rather than your headphones. So here's what you should do to avoid this happening:

1. Swipe up from the bottom of the screen to access the Control Center.
2. Then, in the upper-right area of the Control Center, press and hold the audio box.
3. Select the AirPlay symbol.
4. Make sure Headphones has a checkmark next to it. If a checkmark appears next to another device, touch Headphones to switch to and pick headphones for your sound outputs.

Alternatively, you can simply off Bluetooth or delete Bluetooth devices from your iPhone 8. This deletes all previously saved Bluetooth devices from your iPhone system and disconnects your iPhone from any Bluetooth-enabled devices.

• To do so, go to Settings-> Bluetooth, then press to select which Bluetooth devices to forget or delete.

Restart your iPhone after uninstalling Bluetooth devices to refresh the system and delete the internal memory.