I've only used the video once on my D5100 and didn't have to do anything special on my computer to view it. Sound worked fine.
so it is possible that my camera is defective?
So you have good video but no sound output.
Try asking someone who has a microphone and see if it can record the sound. If it still doesn't work, then it's time to send it to your authorized technician if that is something that you are willing to pay for.