Does the issue occur while using the headphones with different source devices, like radio, cassette tape, CD, or MiniDisc player? Does it occur while playing different music files in the iPhone device?
If audio is being heard from the left side of the headphones only, it may be possible that the audio output is in mono instead of stereo output. Generally, if a device has an output jack labeled EARPHONE, it will be mono, while an output jack labeled HEADPHONE will be stereo. Test a different set of headphones on the device. If sound can be heard from both side of a different set of headphones, then the issue is with the Sony MDR-XB800 headphones and service may be required.
If an extension cable is being used to connect the headphones, disconnect the extension cable and then connect the headphones directly. If the issue is still not resolved, service may be required. Use the following link to setup a repair for your headphones.