Nah, the motherrboard does not have to be THX cert, but the soundcard does, and the speakers.
But, what does THX computer speakers do for you? Very little, that's what. Pretty much THX cert for computers stuff is a gimmick. I have never seen a THX setup on the computer that has not been whomped on by a non-THX setup. Actually, I think 3 pairs of M-Audio monitors, with one left in the closet would make for an incredible computer sound system....one that NO computer THX certified system could even dream of competing with.
Only downside to that is the size of the speakers, and the need for stands.