Remove nib mentions of volume overriding.

This feature never worked properly, was never public and triggered barfing when changing output device.

The change should have been included in dee9e6e4 but Xcode still doesn't search in nibs.  (Or perhaps the moral is "don't use Cocoa Bindings".)
Work around OS X 10.8 issue breaking speech voice changes.

In OS X 10.5 and 10.6, you could change the voice without recreating the world; now you can't (at least, not in the general case).

A similar issue is described here:

http://lists.apple.com/archives/speech-dev/2013/Jan/msg00002.html

To direct speech to an audio device, this replaces AUGraph-based speech synthesis with a speech channel for which an AudioDeviceID is set.  (This API was added in 10.6, whereas the speech AU was added in 10.5, which explains why I couldn't do it this way originally.)