I believe the difference between a vent tube and a trach tube is that the trach tube is inserted below the vocal cords (it may also be smaller). Since the vent tube goes through the mouth and down the throat, talking isn't possible. With the trach tube, the throat isn't obstructed. I don't know about the voice sounding normal - it is probably a little different, but chances are that it isn't too terribly abnormal!

Here's a link to some more information:
http://www.icu-usa.com/tour/procedures/trach.htm