Searched this question myself and found nothing. My dog has lost her voice as well and cannot bard loud or howl anymore. Did you find anything out??
I hope it isn't something terrible. You should really get the pup to the vet. Hope this helps.
There are certain plants that cause damage to an animals larynx that if chewed could cause permanent laryngitis I have seen this in some dogs but mostly in cats as they eat weeds or grass to settle an upset tummy. Is a possibility though.