I'm surprised I haven't found a post on a problem like mines, but here goes...
I have video and audio being carried over hdmi to my tv. I had been using the dvi and audio out connections, but decided to give this configuration a try for a while. It's got it's good points, and some bad, but my main issue is that certain actions will cause the audio to have a very pronounced (and very annoying) echo. I can't think of any one thing that triggers this behavior consistently, but it happens often when switching video (wmp, vlc, even flash) to full screen. I can run down the long list of things I've done to resolve the problem (switching audio devices, disabling/re-enabling, etc), but the bottom line is that any of my results have been inconsistent too. For instance, switching to the realtek audio device and back to the hdmi audio stopped the echo once, but the same procedure produced no results 30mins later.
By the way, I'm running Windows 7 64bit, on a Lenovo D10 motherboard, dual xeon 5460's, 8GB ram, and the hdmi connection is through a MSI gtx 580 lightening.
update: I was able to get rid of the echo this last time by muting the hdmi audio and then un-muting. This is after not resolving it by disabling/re-enabling hdmi audio. One other observation that might be a clue is that I get a momentary black screen (about 0.5sec or less) when muting/un-muting audio (and sometimes when switching between window/fullscreen) since I've been using the hdmi connection. I know I could save myself the headaches by going back to the dvi connector, but I'm forced to use a vga adapter with that output, plus while there isn't a noticeable difference in picture quality, brightness and especially contrast are much better over hdmi, which helps overcome daytime screen glare.