First, check to make sure all cables are fitted completely into the respective jacks - both on the headphones and the audio source.
Make sure audio is playing from the source device when not connected to the headphones.
Make sure the MH40 Mute Button is not activated. The Mute button is located on the right earphone.
Try a different source device to see if the issue is isolated to one or if it persists across any device used with your headphones.
Check your audio cable for signs of wear and test another cable to see if the issue persists.
If you're unable to resolve the issue after trying the above, please submit a ticket, and our support team will do their best to assist you.