you use the front panel connection right? got the same problem... and there are other guys, too. They told me here it was my front panel cable, but I believe it's a poor connection with crossover on the board itself.
As long as your voip programm has no echo cancellation option, your can use the back connections of your motherboard.
cheers
btw I'm running the formula so it doenst matter if the board has some better sound components on the pcb.
my hope is that it can be fixed with a driver update.