When someone starts singing all of a sudden with full enthusiasm, that is 'bursting into song.' There are different words that can be used instead of those three words to indicate the same. One such word is 'serenading' which implies singing passionately or romantically. 'Crooning' can be used to refer to singing soft and soothing tunes. 'Warbling' indicates trilling one's voice in a melodious way. Another synonym for 'bursting into song' can be 'belt out' that means singing loudly and powerfully. 'Chirping' refers to singing like a bird with sweet and joyful notes. In conclusion, these synonyms add variety to the expression, 'bursting into song'.