MacBook – MBP retina + Dell u3011 via HDMI: intermittent screen message about audio

I have a new Mac Book Pro (MBP) retina connected via HDMI to a Dell U3011 display. Every few seconds, but irregularly, the display pops up "MESSAGE Unsupported audio format. Please set the audio output on your audio player to Pulse-code Modulation (PCM) if available"

The message is dead center on the screen and lasts much too long. This is highly disruptive and makes it almost not worth it to use the screen.

I would like to find any way at all to turn off the message, either on the Mac side, the Dell side, or both. I do not need to pipe audio from the Mac to the Dell or vice versa. I'd be happy with a way to turn off ALL messages if it will get rid of this one.

Here is what I have tried. In the Mac's System Settings, I found Sound, the Dell shows up as one of the audio devices, but all settings are grayed out on it.

I have muted the sound on the mac via the function key F10.

I've been to the Apple support forums and not found a solution. I also left a message there.

I've been through the Dell manuals online for the U3011 and not found any way to configure the HDMI in port to turn off audio, and not found any way to turn off the messages.

I've been through all the on-board menus of the Dell, including the sound menu. The only option in that menu seems to be 2.0 versus 5.1. I didn't see a way to turn off sound input, and I didn't see a way to turn off messages.

It could be I've just missed the appropriate documentation or menu item, but I spent considerable time looking without success and thought I'd consult the sages at "AskDifferent."

EDIT: I'm pretty sure the message comes from the Dell firmware and not from the mac because it looks just like all the other messages from the Dell: it's in a non-Mac font, it's at a different resolution, there's no anti-aliasing of the font, etc.