Take great care when swapping sound cards around. It can leave things in a bad state.
If you swapped the sound card and the problem wwent away, then you are on the right track. Were you careful to remove the drivers for the other sound card, before swapping the cards?
DirectX gets a bit wonky when more than one sound card driver is installed. Note, I said 'driver', even if the hardware is removed, DirectX will still run the drivers init routines which can leave things in a bad way.