There is an app called "Notes".  It doesn't exactly allow you to send messages directly to other chumbys from the app itself.   But if you have multiple chumbys registered under one account, then the messages you create and save will sync up with all of them.  No audio though.

There is a Gmail viewer widget. If you set up a gmail account specifically for the chumby, then by using this widget, you would have a way of sending messages to that chumby only. Still no audio though.
I can see where you're going with this idea though. Something that brought the message to the attention of the recipient, and displayed it in big text readable from a bit of a distance, would be a pretty cool thing.
It could perhaps be done by modifying the gmail viewer to just display the latest message in much larger text, and then having a gmail account just for chumby messages.
Then as you were leaving work you could send the chumby an email saying "Leaving now" and those at home would see it and start preparing refreshing drinks and delightful little snacks ready for your arrival.

It seems like the display of the text could be accomplished with an overlay very similar to the method used with clockoverlay.xml. It would be nice to be able to specify how long (how many seconds) to overlay the text as well as to be able to automatically center horizontal and/or vertical. I also noticed another way to very temporarily display text with fbwrite. The only issue that I could see is that I seem to have to do a "killall chumbyflashplayer.x" to re-read the file (unless someone knows another way to do this).

I'm playing with peer-to-peer in the new Flash.  You could network real and virtual chumbys via the user account and get real time messaging.  It would be really slick as long as you didn't need guaranteed / deferred delivery.