"Belt out" is a phrase commonly used to describe singing loudly and confidently. However, there are many antonyms that describe the opposite of "belt out." "Whisper" is one such antonym, meaning to speak or sing softly and quietly. Another antonym is "murmur," which implies a low, faint voice. "Mumble" is another good antonym that suggests mumbling or speaking incoherently. If someone is not singing loudly, they might be "crooning" - a softer and more intimate form of singing. Finally, "hum" is a great antonym for "belt out" which indicates someone singing softly or humming to themselves. When it comes to singing, there are many ways to express oneself beyond belting out a loud tune!