I around a bit getting audio working on a couple desktops here in the office. Most of the engineering workstations are Asus A8N-E or A8N-SLI boards. There’s a jumper block labeled FP_AUDIO on board for the front panel audio. You may have noticed in the past that some computers will disable the rear speaker output when you connect something to the front output, such as to automatically turn off the speakers when you plug in headphones. I find this nice. The catch is that this is usually done by routing the audio to the front panel and then back, using a mechanical headphone switch that allows the electrical path to continue back to the motherboard when there is no connector in the front panel, but opens the connection when you plug in.
There’s some decent specs here showing the connections. Basically, if you don’t have the right type of front panel audio connected, then your rear audio connector is disconnected. Alternatively you can jumper pins 9/10 and 5/6 to force audio back to the rear connector when not using a front panel. I have yet to see this done on any of my boards, but I get the impression that this is default.