Hi Maiku, saw this thread the other day but didn't have time to reply.
Yeah that should all work ok. Why dont you try testing the things one at a time, for example just hook up the line-out first and see if you can get that going. The mic should work too but you need to get the "mic" and the "micp" signals the right way around. "Micp" is a power (or bias) signal and normally goes to the centre section (or "ring") of the mini-stereo jack. These boards work best with the "three wire" type electret micophones (powered).
the sound from my speakers gets noticeably quieter and there's no sound coming from the headphones
Well the "noticably quiter" is due to the impedance loading that your headphones cause. The line-out is not really designed to drive headphones properly (very unforunate this as it would only cost about 10 cents more to do it properly). So there probably is sound comming out of those phones but it's just so low you didn't notice it. Turn off your speakers and then turn all your volume control sliders to max in the windows mixer, then you'll hear it but still not very loud.
BTW, I noticed you've got a line-in header too. I think that's a new one, I dont recall my previous k7s5a's having line-in on the front audio header.
Anyway I just checked with my most recent k7s5a and sure enough it's got line-in too. But there's a bit of a catch with it. I just gave it a quick test and no go. Intesting solution though, just plug a "dummy" jack into the rear line-in socket and then the front one will work. Must be a shorting switch on the rear line-in to keep it quite when not in use, the designers probably overlooked it's impact on the front audio, I've seen ECS/PcChip make this mistake before with front-mic also.