I will hook up my laptop to our tv via HDMI and every now and then I have to go into control panel to select the sound source. Either "Speakers and Headhones" (how it is listed in my computer... OR "HDMI Sound"...
Usually it is automatic for my computer to know what is going on as it will switch sound sources once I connect or disconnect the hdmi cable.
When it doesn't play sound on my computer after I disconnect the hdmi it is silent of course, and I have to go into Control Panel to make the switch.
Maybe something like that happened to you? Either way, go into Control Panel and check there if you haven't already.
[This message has been edited by SeptyTwo (10/4/2013)]