Realtek Digital Output Error

The digital output that you are seeing is the optical S/PDIF connector, usually on the back panel somewhere. Optical connectors have no feedback mechanism which means that there's no way for a device to tell whether or not an optical cable is connected. As a result, it will always appear as being connected; this is standard behaviour and you can safely ignore it.

Using your headphones and your 2.1 speaker system is fine, but you may have a hard time using them both at once. Some codecs support separate front panel and back panel audio sources, while others simply shunt the front panel with the left/right channel on the back panel.

If they are treated as separate sources (unlikely with Realtek codecs, but some Creative Labs cards do support this), then you can simply plug your headset into the front panel and your speakers into the back. Finally, select which one you would like to use either in game (if supported) or by setting it as the default in the Windows playback devices.

If they are not treated as separate sources, you can try connecting them both at once, and simply turn off the Corsair system when you don't want to use it, and disconnect your headphones when you don't want to use those. If both are connected simultaneously you may experience a slight decrease in audio quality.