I had a similar problem with no sound over HDMI with a new Acer Aspire 7740. I'm posting this here in hopes that it may help others.
My setup is an Acer Aspre 7740-5691 with Windows 7 Premium 64-bit. I connected the HDMI port up to my LG HD set and while I saw the video but there was no audio other than through the laptop speakers. If I look in Control Panel / Device Manager / Sound,
video and game controllers I see two drivers:
Intel(R) Display Audio (v6.12.0.3037)
Realtek High Definition Audio (v6.0.1.5969)
If I right-click on the speaker icon in the system tray (lower right corner of the screen) and select Playback Devices I see two devices:
Speakers ( Realtek High Definition Audio ) default device
Reaktek Digital Output ( Realtek High Definition Audio )
Even if right-clicked in that screen and selected Show Disabled Devices I still only saw those two devices. I right-clicked on Realtek Digital Outpout and chose Set as Default Device, hooked up the laptop to the HDTV and still no sound. Then while the
laptop was still connected to the HDTV I right-clicked on the speaker icon again and chose Playback Devices and now there was a 3rd device:
42PC1DA-UB (Intel Display Audio)
My HDTV is an LG 42PC1DA. I right-clicked on that device, chose Set as Default Device and lo-and-behold I now had audio over the HDMI port! When I disconnected the HDMI cable that Playback Device disappeared.
So it appears Windos 7 and the Intel Display Audio driver are dynamically creating the playback device as needed. I'm just glad I happened to look at the playback devices while it was connected or I never would have solved my problem.