Shielding motherboard sound from electromagnetic noise can indeed be a challenging task. Even if a motherboard has fancy audio chips, it doesn't necessarily mean that the analog outputs will be pristine.
It is worth pointing out, that in this day and age of switching power supplies and class D amps, there are many "non computing" sources of noise, and the challenges can and have been overcome...
Doing so on a motherboard, is no more difficult - just requires a bit more time and effort at the design stage, and a very marginally higher manufacturing cost (maybe)....
So there is no excuse for 2nd grade audio output on anything but the very cheapest basic motherboards .
