ATI 5870 audio issue - HDMI audio switching off

[Horrgakx]

Hi all. If anyone can help with this I'd be grateful...

I have an ATI 5870 card which feeds audio and video via HDMI into a home cinema amp. The issue I have is that some games stop outputting audio.
As an example, Dangerous Waters (recently bought over Steam) has audio in the menus but when the game goes into 3D mode the HDMI light on the amp flashes and goes off as it's not receiving a signal.

There are no audio options in these games that gives a clue and I've tried different audio formats (eg. 16-bit 32kHz and 24-bit 96kHz) but it makes no difference.

Hardware spec;
Onkyo TX-SR605 amplifier
Win 7 64-bit (fully patched up)
ATI 5870 (driver version 11.3) [latest])
ABIT IX48-GT3 Socket 775
Intel Core2Quad Q9400 (2.66Gh)
Corsair 650W PSU

Any ideas please?

 

[Horrgakx]

Update - in the SOUND options within the game it reports;
"Sound card reporting no 3D hardware buffers. Could not create hardware 3D sound".

Where do I start to look?