Sound only from the right speaker

There is a setting that changes the output format of the audio. It is my suspicion that the left channel of your audio is being routed to your subwoofer. Windows may not be autodetecting your speaker configuration properly and might be sending a stereo signal to your speakers incorrectly.

Try right-clicking the sound device and clicking Configure Speakers, instead of double-clicking to get to Properties.

A configuration wizard should appear, starting at the "Speaker Setup" step. It should let you change the output format of the sound. Choose the one corresponding to your setup. For instance, this is what I see when I do it on my laptop:

Also, make sure that you are hooking up your cables properly behind your computer! I have a 5.1 audio system hooked up to my computer at home, myself. My motherboard supports up to 7.1 surround, and so not all of the audio output ports are hooked up. Make sure the color-coding on the speaker cables and your back panel connectors match. In my case, the orange and pink-labeled inputs are very similar and, if you aren't careful, can be easily mixed up.

Have you tried disabling the mobo soundcard from BIOS and running strictly off of your PCIe one?

Are you getting full range sound from your subwoofer instead of only low frequencies? Can you hear voice and system sounds, or is it playing correctly?

Since your getting stereo from the 3.5mm jacks, sound is being processed correctly into analog, which is actually one degree further than using the SPDIF which is full digital.

Have you tried changing your SPDIF output mode?

Does your speaker setup have different options for surround output modes? Dolby, Quadraphonic, etc....

What model of edifier speakers are you using?

Root cause : The cable for the LHS speaker is broken. It might have broken at either of the ends.

Try connecting your speaker to another audio output - ipod etc. If it does not work, then this diagnosis is right.