The voice isn't totally redundant, you still use it for getting their attention, ie if you have sent them through a tunnel and then changed sides you could use it whilst they are in the tunnel, so they know where to go as they come out and you get a sharper turn etc. I do usually still use my voice anyway, I find it hard not to!!
For a tunnel under an a-frame; if your voice says tunnel and your body is saying a-frame - the dog will go over the a-frame. A lot of what the dog reads from our body language, we are not even aware of. That's why it is very useful to get someone to video us, so we can see just what we are doing.
So for instance, if the dog is on my left coming head on to an a-frame with tunnel entrance on the right, you could turn your shoulders right and send your dog towards the tunnel. Although alternatively what I might do is turn slightly towards him with my right arm tucked into my waist, thus dropping my right shoulder; which will pull him towards me, then immediately pick up with my left arm using an underarm motion sending him to the tunnel (pehaps similar to throwing a bowling ball). Timing and position are the key! Interestingly dogs pick up on the signals we unconsciously give, so before we have actually done as above, he will have it sussed.
If you can imagine your body language from the dog's perspective had you wanted the dog to go up the a-frame, would have been very different, much more upright, and to the left.
Working as a partnership the dog doesn't have to be looking at you to read your body language. Try it and see how you get on.