I don't have an EPIA so I'm not sure exactly what "switched to six channel means", but I don't think it has anything to do with mixing the 3 audio ports. What I can tell you is that channels only apply to a single line at a time. I.e. your wave out line will have 1 channel if its mono, 2 if its stereo, 4 if front and rear that are both stereo, etc. Half of the channels would never apply to input and the other to output though. i.e. you would never have a 4 channel device with 2 channels for stereo wave out and 2 channels for stereo wave in. Also the channels can map to a single or multiple connections (of the same type) on the physical sound card even though it only shows up as a single line input with multiple channels in the mixer. This is how the windows mixer works and the nomenclature it uses.
I used an external sound card cause the onboard sound on my motherboard sucks and I'd rather run a long USB cable to the front of my car from my computer in the trunk and then a short analog audio cable to my head unit then just a long audio cable since USB is less prone to interference.
Originally Posted by arkadiyp