There is a lot of embedded media content in web pages these days. If you have multiple browser sessions (windows) or multiple browers (IE and chrome and Firefox) they all have their own individual audio "lines". Depending on your OS, this should work:
1) Right click on the audio control in your task bar. This is the little speaker icon next to you clock (if it is displayed).
2) When the menu appears select 'Open Volume Mixer'.
3) You should now see three to a lot of little sliders. Find the one whose icon resembles whatever browser you use (IE, Chrome, Firefox, Safari, etc). Drag the slider all the way down. Or, just click on the icon below the slider. This will mute (if sound is coming out) or enable (if they are muted).
If this doesn't work post back.