It is true that you are not allowed to use your voice in dressage - even if the judges cannot hear you, they can see what your mouth is doing, and will mark you down.  I know that it is good to talk to a horse normally, and too many people do not, but in a dressage test it is a no no.  Responses from the horse must come only from the rider's body aids, namely hands, legs, seat and back and at the moment they are asked for, though some horses need a bit of verbal encouragement.

I would just like to say that any voice command will get you marked down, including clicking (I write at dressage competitions and the judge i write for has marked a rider down for clicking, as well as general use of voice).

Yes, this is correct. Dressage is all about control and using the aids properly. i alwys use my voice when riding, but have ha to train myself into using the aids more effectively. The dressage is to show your riding capability and how much the horse is willing to sumbit to you and listen to you.  Using your voice says to the judges that this has to be done,, what you can do (i think),, is use a click of the tongue though, i find this works just as well, and you;re more likely to get away with it,, as it is less blatant.

Good luck for next time though

(: Xx