Note: This post is relevant to all models of REAL BLUE/BLUE NC.
Possible Cause 1: The 3.5mm jack cable is not connected properly.
Solution: Connect the 3.5 mm jack cable to the headphones properly.
For a Windows 10 computer, connect the 4-pin plug to the headphone output of the computer.
Possible cause 2: There is no Bluetooth® connection (only with BLUE / BLUE NC).
Solution: Establish a Bluetooth® connection. Go within range of the external player.
Possible cause 3: Volume set too low. Increase the volume on the external player and/or the headphones.