It is possible that your headphones have not been paired correctly and that is why only one is emitting sound. To fix this, follow these steps:
Important: Make sure that the coloured strips that cover the charging contacts of the base are removed and that they have been charging the battery for a while.
1. On your mobile: Remove previous links Settings-> Bluetooth -> (delete, forget ...).
2. On your mobile: Turn off the Bluetooth.
3. Put the headphone base to charge.
4. Put the earphones inside the base with the cover closed and leave them charging for more than 2 minutes.
5. Take them out of the base and wait a few seconds. Press and hold the touch panel of each earphone for 11 seconds.
6. Put them in the case for a few seconds. Remove both earphones from the case to automatically pair them with each other.
7. They should start blinking for a few seconds and then one will stop blinking, that means they have been paired.
They can now be linked to the phone.
7. On your mobile: Turn on the Bluetooth.
8. Go to Settings-> Bluetooth, search for the earphones to connect.