it is the analog outputs you need to use. they are the left and right outputs.
if you want to have output from your onboard card too, either get a small behringer mixer (they're cheap) and put the outputs of both cards to that then the master mixer outs to your speakers... or connect the line output of your onboard card to the stereo monitor input on the UX2. that's just like an aux input which it will mix to the outputs of the ux2 as well as any other audio going through your UX2...
if you're stuck, next thing is to look at the manual, also search on this forum. many people have asked the same thing and got good answers already.