No help (yet) I'm afraid, but I just bought a PC equipped with P9X79 motherboard, connected it to my Z523 speakers...and it didn't work. Running W7 Pro. Have also tried switching between AC97 and HD audio i BIOS, updte drivers etc. The output level indicator for speakers indicate that there's being processed music, but no sound whatsoever.
Will post again if I'm able to solve this.