That's a pair of good points! If in device manager you see two audio devices, disable one. Usually you would have onboard audio (AC97 at a guess) and a soundcard - disable the onboard - your sound card will be higher quality.
I once had no sound from the left channel, I updated drivers, tried swapping cards, reinstalled the software, the lot. In the end, it turned out I had the fader turned round so one channel was out!
