I have setup video playing with audio perfectly working via HDMI (I used both "bare metal" video playing with omxplayer and specialized application for raspberry called Screenly for digital signage). Both worked OK.
All of the sudden audio via HDMI stopped working. Video worked OK. Panic
After about an hour I've switched SD cards, started hello_audio.bin both to analog and hdmi output - it worked OK.
Then started playing videos again and audio via HDMI started working.