2 Channel is all you can get with the AUX in on that unit.
Now amount of settings in the Realtek panel will change it.
to get 5.1 you need one of the following.
HDMI
HDMI was introduced to bring video/audio to your TV, but has become big for audio only use as well in some setups(one cable to rule them all). This is a 100% digital setup and can pass 5.1 and higher channel sound with the right hardware.
Optical Audio/TOSLINK
Optical or TOSLINK is a fiber optic cable for carrying audio. With the right hardware surround sound can pass over this medium. It also has the advantage of being ground loop proof by design.
Coaxial This is another
SPDIF cable similar to optical, but with copper instead of light transmitting the data.
Coaxial is another 100% digital cable that can carry 2 or multi channel audio with the right hardware. It looks just like a normal RCA connector on most systems
Analog Audio
The old fashioned way of getting it done. Each channel can have its own RCA type plug or 2 channels per 3.5mm TRS jack(I am not including
balanced audio here because it is outside the scope of this post.).
This is used quite often on computers, but has lost favor on home theater equipment.
Analog audio has no content protection making analog in general not as common on newer HD formats that want 100% control.
Analog audio can work very well, but also can have more interference since unwanted signals that get into the cable can be amplified and played back on your speakers. Running shorter cables helps reduce this risk. Analog is not as bad as some make it out to be and is very diy friendly.
With your current setup, your best be would be to see if you have any kind of upmixing options on your DVD player(acting as you amplifier by the look of things). This is not true surround, but some users still like to have all speakers in use.
edited for a bit a of clarity.