I would like to hear vocals at certain points in the game where it fits, though this thread, I believe, is about a theme song based on vocals. For example, a certain tune with vocals could be important for completing a quest. It would also add a lot of immersion to the game, where travelers would pass on songs they heard, etc. Perhaps they will come up with some sort of native tongue to Hydlaa/Yliakum, which music could be sung in? I think language in the game hasn\'t really been touched on.
As long as the quality is as high as the other PS songs, vocals would be fine, but I do not think they should be part of any main theme (that would be repeated), but should be attached to specific events.