There are many synonyms for the phrase "sing low". One common synonym is "whisper sing", which refers to singing in a soft and gentle tone similar to a whisper. Another synonym is "croon", which describes a soft and melodious way of singing. "Hum" is another synonym for sing low, which involves singing without words and creating a continuous gentle sound. "Murmur" is another synonym that describes a low singing technique, creating a soft and indistinct sound. "Mumble" and "mutter" are also synonyms for "sing low", which both refer to a quiet and non-vibrant way of singing.