Please make sure you are using an updated version of either Chrome or Safari, ensure that your external speakers (if you're using them) are connected and the volume is turned up and that you do not have another tab open that is also playing sound.
Please also double-check your computer is not on mute and that any external speakers are completely plugged in. If that still doesn't work, please click the media player button (the red button in the tray) and make sure that it is not on mute.