If you are experiencing sound issues with your Fnatic headset, please try out the following solutions. 

First, check if the sound volume wheel on the cable is properly set up:

If the issue with the low sound persist, please check the Sound settings in your Windows:

Under the 'Playback' tab, right click on an empty space and enable "Show Disabled Devices" . Then locate 'Sound Control Panel' in top right:

Look for a device that is disabled and re-enable it, as this may be the connector for the REACT headset. Disable any other devices.
Alternatively, you can test out the headset with another PC/device. 

If your headset volume is low during the game, while your game is running, please make sure that the volume is properly set up in the volume mixer: