When using the HDMI audio, the video card is using it's own built in sound device, not the PC's sound device. If the sound works OK with the regular PC sound (Realtek) and PC speakers, the problem is in the Video Card's sound system.
Youtube uses Flash and that has known to be iffy at times. Make sure you have the latest version of Adobe Flash. Some web pages with embedded audio use Windows media player extension and with the HDMI audio that may also be a problem.