A roomful of people conversing by American Sign Language isn’t so silent. Signing is a physical, energetic act. It sometimes needs a grunt for emphasis. It can lead to an eruption of laughter. But in calling this Friday night gathering of hearing and hearing-impaired Jews in their 20s “Silent Shabbat,” the organizers had two things in mind: They wanted to connect everyone through the use of a common language, and they wanted the hearing to experience a Shabbat where meaning did not come from sound.