I searched all over the forum and all over the internet, and I see people grumbling about this error, but nobody seemed to know how to fix it. Well, I kept tinkering around with things and I eventually figured it out.
All my A/V devices (360, PS3, Wii, Samsung DTB-H260F) were connected to my Onkyo TX-SR607 receiver with HDMI, which was set to output to my Sharp Aquos LC-52E77U. And, although the receiver was set to "Audio TV Out" off (and even with it on), the TV kept popping up this error:
"Incompatible audio signal has been received, please check the output signal."
It only pops up for a few seconds, and only when I would switch devices or resolution or whatever. Still, pretty annoying. Connecting the devices directly to the TV via HDMI caused the same error - the only time it didn't happen is when there was 2ch PCM involved.
Unfortunately, unlike every other TV I have ever owned, the Sharp doesn't have a menu option to disable audio. Muting didn't help, either.
The solution? Set the audio volume to 0. No more error!