Can hypnotherapy help relieve the symptoms of spasmodic dysphonia?

Doubtful. I am unaware of any study showing improvement.
I believe it can. Hypnosis has been used for this disorder. I personally have not treated such a case but have had success with similar conditions such as torticollis and stuttering. There are many treatments we use in medicine without a scientific study proving the value. It would be nice if one had the time and funds to run studies on everything but who does.
No. I am not aware of any good data on the use of hypnotherapy for spasmodic dysphonia. The most effective treatment is the use of botulinum toxin.