Okay, first of all, it requires the Microsoft Speech SDK V5.1 (Not the 1.0 .NET Beta SDK), and the add the preprocessor definition FS2_SPEECH for it to be compiled.
It's not done yet (And I really couldn't do any work over the last month, and I won't be home for another week), meaning that there are still a few loose ends to tie, like the fact that speech in briefings will continue (for the current stage) even if you move over to ship selection or weapon selection, or that in-game speech (That's right, in-game messages that don't have a wave file attached also get text-to-speech) doesn't pause when you go to a menu or hit escape. Those are simple stuff to implement (and the functions to do so are already there), but I just need my home computer to implement them, or perhaps RT already did, and I didn't have the updated files to integrate.
Okay, now to what it DOES do. Every command briefing, red-alert briefing, or in-game message that doesn't have a wave file attached to it has the Microsoft text-to-speech algorithm go over it. I won't lie to you. The voice is ugly, monotonous, and beyond that, also VERY sensitive to proper quotation (So put as many commas as possible). Why is it good, then?
You don't have to read anything, and you don't have a chance to miss messages (Though, I'm not sure that it does training messages).
Quite a few times, in user-made campaigns, I missed sometimes important messages in the heat of battle, simply due to the fact that I couldn't pay attention to the upper right corner of the screen for enough time to read the messages.
I think that the fact that this problem will be gone without the need for every campaign to do voice-acting is an advantage that outweighs the limitations of the feature.