terminal command say produces error
A little Googling suggests that  it's a new problem in 10.8.4. A couple people in this MacRumors forum thread have experienced the same, as has the author of this blog post. No indications as to how to get rid of it however.

I haven't seen any error when testing say, so it may be some combination of hardware that causes it, but if the sound output is as expected, I wouldn't worry too much about it, and it will probably be fixed eventually.

If you're using say in shell scripts and need the Terminal output cleaned up a bit, you could always pipe it through grep: say test | grep -v "MTBEAudioUnitSoundOutput::PropagateProperty Error -10851".