Adverb
without a sound
Source: WordNetYou're my sister,” he said finally. "My sister, my blood, my family. I should want to protect you”-he laughed soundlessly and without any humor-"to protect you from the sort of boys who want to do with you exactly what I want to do. Cassandra Clare
Overall, the library held a hushed exultation, as though the cherished volumes were all singing soundlessly within their covers. Diana Gabaldon
he stood up soundlessly and speechlessly and glided across the hallway and through a door Source: Internet