Realtek ALC889 audio, audio fine on S/PDIF, nothing from rear output

I'm having a strange problem in my HTPC. Audio is coming perfectly fine out from the S/PDIF coaxial output, and it's normally connected only that way to the receiver, but now for other reasons I also need to connect it using a 3.5 mm to a small mixer. So in the sound manager I changed the output to the "Speakers", and in fact the Realtek sound manager detects that a cable is plugged into that output and shows me the dialog where it's selected. Problem is, nothing comes out.

I know it's not the 3.5mm to RCA cable because I plugged in my phone, played some music and I can hear it just fine. But the rear speaker (green) output puts out no sound at all, as I also tested it plugging headphones. On the Sound Windows panel where you can select the playback device I can see the volume level move up and down when I'm playing any media file, as well as in the Volume Mixer. If I switch back to the S/PDIF output and change the input in my receiver, audio is coming just fine.

I haven't tried the front outputs because they are not connected. The motherboard has HD Audio but the case has AC97 connectors, so I never cared to connect them because I don't use them. I know the rear green output worked at some point because I remember having a pair of computer speakers connected to it a few years ago.

This is a Gigabyte GA-890FXA-UD5 (rev. 2.1) motherboard with an AMD 1090T and 16 GB of RAM, with Windows 7 Home Premium. The installation is about two months old with the drivers from back then, and the latest audio driver on the Gigabyte website is from 2012, so this driver is the latest available. I tried rebooting the machine just in case, but it didn't help.

Any idea what this could be? Did a power surge selectively fried the analog part of the audio chipset? It is connected to a surge protector. I built this computer myself and I'm always very careful to tie every cable and not leave them loose inside the case.

Thanks,

Sebastian