The phrase "belts out" refers to singing or shouting loudly and confidently. The antonyms of this phrase would be expressions related to silence, whispering, or keeping quiet. Some possible antonyms include muttering, murmuring, speaking softly, or even remaining silent altogether. In music, the opposite of "belting out" could be a delicate or tender performance, characterized by a controlled singing technique and an expression of vulnerability rather than power. Regardless of the context, the antonyms of "belts out" suggest a reluctance to draw attention or create a strong and assertive impression, instead emphasizing subtlety, discretion, and selectivity.