Help! My Sound tracks reverse!

My Sound tracks(or be called sound channel?) was reverse suddenly! I mean, the sound should be sounded from the left speaker, but it's sounded from the right speaker now! And the sound should be sounded from right speaker, it's sounded from the left speaker(sorry for my poor english)!! So I test by many ways: change the speakers; change the soundcard; check the line connection(of course they're all connected correctly); try many soundcard's drivers; even tried many OS(win98/win2000/winXP)...and the problem are still there! I have been in consultation with many friends, and asked it at many forums, but no solution. Now I suspected that the point is electric circuit of the mainboard! what do you think? Anyone could HELP me?