Have you tried uninstalling/reinstalling the latest drivers for your radeon? Make sure the software is also up to date for your onkyo and you have the latest intel chipset drivers.
Yes. To no effect. It is acting as if the input termination into the amplifier is getting into an indeterminate state when the amplifier switches; like the signals float and this screws up the operating state in the hardware (The ATI) card. What is odd is that it only impacts the audio side of the system and not the video.
I may have to go digging around in the HDMI 1.4 hardware spec and see if there's a hot line switching issue.
I'm having a very similar problem with a HD6870 and a Yamaha receiver where I loose video and audio. But what I just found out worked (only once so far) is that unplugging the HDMI and plugging it back in on the back of the card worked and reset it.