Are you sure? Because in researching your issue for you, I have seen post after post all over the net that Hulu doesn't stream in surround sound. Only stereo. Are you sure your receiver isn't doing some pseudo surround sound thing?
You shouldn't have to. I just tried my ATV Hulu app and I got stereo just fine. I did have to change a setting on the receiver though that keeps getting overridden somehow. Maybe you have a similar situation? Or maybe you have an HDMI cable not seated properly or going bad? Or a port on the receiver going bad?