It seems for some set of users the audio device used to become unavailable after restart even though it was working fine when plugged in. This resulted in no sound when an audio device was connected to HDMI section of a computer running Windows 7 and Windows Server 2008 R2.
Good news is that there is a hot fix available which you need to manually download and apply to your system from here. Like I said this was happening only with certain users hence it was more limited to a particular set of hardware which had timing issue during audio initialization. You may have to restart your Windows after the installation is complete.
Till now the only workaround was to restart the audio service to detect the device again or use the audio troubleshooting section. To download the fix you will have to manually raise a request by following this link which is also available right under the title of the said page. However this hotfix is available only for x86 based system so if you have a 64 bit, you might have to wait for sometime to get this.
Now before you go and download this hot-fix, make sure you had this kind of symptoms and issues because any incorrect installation done can cause damage to your system which means you will waste a lot of time installing your current system again.
So make sure you do have a restore or backup point ready to revert back and you had used the Audio troubleshooting once to see if the error is not a minor one. This will help you simple problems like Audio Muted or not set as default device which is a common issue when you switch Audio Output in a computer.