A soporific voice can be defined as one that is soothing or monotonous causing drowsiness or sleepiness. Synonyms for this phrase could include lulling voice, hypnotic tone, or somnolent inflection. Other terms that could be used to describe a soporific voice include droning, hypnotizing, or numbing tone. Anodyne or tranquillizing voice could also be used to describe someone with a voice that has a calming effect on others. Additionally, the phrases calming voice, peaceful intonation, or serenading tone could be used to describe someone with a soporific voice. Whether it's for relaxation or to help someone fall asleep, a soporific voice can be a valuable asset to have.