I used a HDMI cable, and I have made the speakers my default device and I am using it. When I play a youtube video, the green sound bar will go up and down to show that it is registering the sound. However I cannot actually hear anything, even when i turn the monitor and the sound to 100%. I have tried troubleshooting it, I have the option of clicking on the monitor speakers, and then after it has troubleshooted it, it says that the device appears not to be plugged in. I don't know what to do. This has worked before, and my headphones work perfectly. I have windows 8 and a nvidia graphics card if that helps. What can I do to fix this?