CEDAR RAPIDS — The voices of both sides were heard, via American Sign Language and spoken word. Members of the hearing and deaf communities met in a public forum Tuesday to discuss the controversy surrounding Theatre Cedar Rapids’ upcoming play, “Tribes,” now in rehearsal. When no deaf actors came to auditions in late August, the community theater cast hearing actors in the key roles of Billy, a deaf young man who was raised in a hearing world, and Sylvia, the young woman he meets who is losing her hearing and introduces him to the deaf community — a new family, or tribe, in which he finally finds his identity.Identity is the heart of the local deaf community’s grievance over the casting.