TTS is an xPL thing, not an xplrionet/rio thing. So you need to send an xpl messages to get it to announce something. Using scripting in xplhal it would be possible to, say, pick up a now playing message and announce it back.

With regard to voice files these are for use with other xpl app's to allow voice control of xplrionet. Basically it tries to create voice reco safe files. This is not really used yet, but works, but does need further work.