Although the headphone jack for the dictation transcribing unit is monaural (one channel) and not stereo (2 channels), the headphones that come supplied with the transcriber are binaural, meaning that you should still be able to hear the monaural sound from both sides of the headphones. However, one side of the headphones is active with a single speaker attached by a wire where the other side of the headphones receives the sound passively through a plastic hollow tube. Therefore, the sound volume will be louder on the active side and softer on the passive side.
Sony® does have an optional headphone set available that has an active speaker built-in on each side. The MDRU10M headphones may be available for purchase from Sony Parts and Accessories Sales. Go to Parts and Accessories.