I have the same issue, mine is with a VISIO TV.
I have gotten HDMI sound to work on a few occasions but have pretty much given up trying as it just seems to be design flaw and is not a good long-term option.
I've decided to install a sound card and hopefully that will be that. I don't have a S/PDIF port, so have not been able to try that option to run the audio over an optical cord.
When I did get it to work, I "cycled" the on-board audio off in the bios, rebooted, shut down, switched it back on and on the second boot, my realtek control panel allowed me to set it as the default device. When it worked, it worked great.
However, this process worked maybe three times out of several dozen attempts. It seems to follow no rhyme or reason. After researching forums and tech support for weeks, I feel like I'm pretty informed now on HDMI cable audio out from a PC. The conclusion, it pretty much doesn't work. There is the newer HDMI Standard 1.4 / 1/4a. I don't know if it works with audio out any better or not.
My motherboard includes this note regarding HDMI Audio,
"HDMI 1.3* certification, support Full HD 1080P
HDMI is able to provide up to 5Gb/s video transmitting bandwidth and multi-channel high quality audio all through a single cable. Able to transmit superior, uncompressed digital video and audio, HDMI ensures the crispest rendering of digital content up to 1080p without the quality losses associated with analog interfaces and their digital-to-analog conversion. GA-880GM-D2H has obtained the latest HDMI 1.3* certification, able to support Full HD 1080p multi-media and HDCP (High-bandwidth Digital Content Protection) which allowing Blu-ray/ HD DVD and other protected contents playback.
* HDMI 1.3 certified in video spec only."
My hardware is a GIGABYTE GA-880GM-D2H Board with a 4250 video chip and Realtek Audio
Audio
* Realtek ALC888B codec
* High Definition Audio
* 2/4/5.1/7.1-channel
* Support for S/PDIF In/Out
* Support for CD In
Good luck to you!!