I'm not saying this to be a smart ass, but your sure the slider I mentioned before isn't all the way to the right (at low or no volume). Something really odd must be happening. if you uninstalled the software and then reinstalled it, the slider should have reset itself to the left at full volume for the training voice.
And through all this the motor volume works fine. If the slider isn't the issue, I think the next step might be to update the sound drivers.
I just re-read your original post. if the slider doesn't work, and the sound driver updates don't work, than another, but much longer approach might be to roll back your updates. I'm not sure if this can be done. The prospect of uninstalling and reinstalling would be a real drag, but it may prove that after your re-install (without any updates that the training voice still works). Of course if this too fails to correct the training voice, I'd be suspect that something else is going on.
I'm stumped, maybe a call to Horizon to assist. Oh and try this link too just to be sure it isn't a hardware issue:
http://www.fsone.com/Support/tm.aspx...key=soundo