Make sure the headphones are turned on
Check the condition of the audio cable. If the plug or any part of the cable has been damaged, that could be causing the problem
Make sure the headphones are connected properly. The headphones may be connected improperly or the connection may have come loose. Remove and reseat the headphone connector on both ends
Make sure the audio device is on and the volume is turned up
Try a different audio device. The problem could be with the audio device to which your headphones are currently connected
Toggle the Hi/Lo switch to ensure it is set properly. If the position of the Hi/Lo switch is in between settings it could result in no audio
Try the additional audio cable that was included with your headphones (QC®15 headphones only, purchased after June 2011). If the issue is only specific to one cable, that cable should be replaced
Check the balance level on the source. If the source has balance level adjustment, it may be adjusted incorrectly. Make sure it is set to a middle level
Make sure the headphones are connected properly. The headphones may be connected improperly or the connection may have come loose. Remove and reseat the headphone connector on both ends
Try the additional audio cable that was included with your headphones (QC®15 headphones only, purchased after June 2011). If the issue is only specific to one cable, that cable should be replaced
Turn off any audio enhancement features on the audio source