The antonym for "plaintive" is joyful or cheerful, while the antonym for "cry" is silence, calmness, or serenity. So if we were to describe an opposite phrase to a plaintive cry, it would be a joyful silence or a cheerful calmness. Instead of a sorrowful, mournful, or melancholic wail, it would be a delightful, lively, and contented silence. The words peaceful, tranquil, and relaxed might also be used to describe the antonyms for "cry" in this context. In any case, the contrast between a plaintive cry and its antonyms is a stark one - happiness versus sorrow, laughter versus tears, joy versus pain.