Follow these steps to troubleshoot the issue:
Install the Sony | Headphones Connect smartphone app, then change the volume using the app.
NOTE: On a device that can be linked with the headset, you will have the ability to change volume level of the headset using that device. You will not be able to change the headset volume on a device that cannot be linked with the headset.
- Adjust the volume on the connected Bluetooth device.
NOTE: If you are in a call and need to adjust the volume, you will need to do so on the connected Bluetooth device.
If the issue is not resolved after following these steps, delete the pairing information for the headset from the Bluetooth device, and pair them again.