We found one dictionary that defines the word sing softly:

Phrases: sing softly and sweetly, more... ▸ Words similar to sing softly ▸ Usage examples for sing softly ▸ Idioms related to sing softly ▸ Wikipedia articles (New!) ▸ Words that often appear near sing softly ▸ Rhymes of sing softly ▸ Invented words related to sing softly