I have the Edifier S760D too for gaming and movies and sometimes music, without a soundcard. My onboard solution is so good, it does not need any soundcard. So, it depends on your motherboards sound quality and what you expect from it. Todays onboard sound is much better than on old days and if you have a premium board, then there should be no need for a soundcard, unless you have special needs. I use both, digital and analog inputs and I cannot see any difference in audio quality.
Coaxial cable is good, if your pc (soundcard or onboard sound) does the job on decoding the digital audio and just sends the final signals through the cable to speaker. Optical cable is good, if you want pass the digital audio from your pc without doing anything with it directly through to the speaker. Then, the speaker will decode that digital audio and takes full responsibilities. This may or may not be better than your pc's audio solution.
But with optical/digital inputs, you usually have only 2 channel. There are some different standards to compress 5.1 audio channels into 2 channel, like DTS. Your motherboard or soundcard needs to support DTS Connect, to compress it live. And if your speaker supports DTS (that Edifier supports DTS), then it will be able to decode that compressed DTS 2 channel sound to the original 5.1, so you get surround through optical cable/digital inputs. That offcourse works only, if the driver is setup correctly. On my Linux system, there is not DTS Connect as I know, so I cannot use it. On Windows, I had to enable DTS Connect somewhere in the settings. With coaxial inputs you don't have all these "problems", because your pc does everything.
I use that 5.1 surround with coaxial and only switch to 2.0 digital, if I want hear music without doing anything else. My motherboard is ASRock Z97 Extreme6 with 7.1 CH HD Audio with Content Protection (Realtek ALC1150 Audio Codec), Supports Purity Sound™ 2 & DTS Connect.
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