Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
soundlessly is an adverb meaning "without a sound". It carries one recorded sense.
without a sound
“he stood up soundlessly and speechlessly and glided across the hallway and through a door”
Definitions, synonyms and semantic relations for soundlessly come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. .