sound quality has become such a generalized term... how far are you willing to take the sound quality? do $500 interconnects sound reasonable?
1. the single core atom is going to be a little faster/better then the via board, but depending on the software you use, it might not be very responsive.
2. overall, i would personally pick the asus board because i prefer to have too much power then not enough.
i built a carpc specifically because of the audio manipulation i could do with it-- i started with a atom board, but found that it could not keep up with my audio processing needs. so i got a 775 socket bard with a dual core pentium, and that has met all my needs from a carpc..