As you are hearing some audio, the card seems to be working OK. Make sure you have your speakers connected correctly according to your setup.
This shows the options available.
Did you disable your onboard audio in BIOS? It probably isn't causing the problem but is better disabled.
Videos as seen on YouTube are in .flv format and uses the browser's Flash plugin. You may need to activate the sound option to hear the embedded audio.
This will explain how to do that.
Another possibility is that the sound is actually muted on the site's player. Look for the volume control slider at the bottom right. The icon looks like a speaker. If it's red, the audio has been muted. Click on it to un-mute it. If it isn't muted, make sure the volume is set to maximum.