You can the adjust volume of each speaker for analog sound only. Follow the steps below for your version of Windows.

Windows Vista

Go to the Entertainment mode > Click the ‘Speakers’ Icon (the first one on the main console) > Click ‘Advanced’

Right Click the speaker icon at the system tray > Playback device

Right click the 'Speakers' > Properties > Level tab > Balance button

Windows XP

Right Click the speaker icon at the system tray > X-Fi Play Control > Speaker Volume