4850 audio over hdmi

i managed to get audio working over hdmi with my 4850, but i'm having a problem i haven't found elsewhere. if i change the source on my samsung a650 from the pc (to watch tv or play xbox, for example) when i switch back to the input from my computer, the audio is gone. the only way i have found to restore audio is to reboot the computer, which is a bit of a pain.

then, of course, audio is perfect until i switch sources again. i have tried disabling the onboard sound on the video card, but this didn't seem to solve the problem. any idea how to fix this? thanks!