There are several synonyms for the phrase "sing out," which refer to vocalizing loudly and confidently. One option is to "shout out," which may be used when there is excitement or urgency involved. Another synonym is to "yell out," which is similar to shouting out but may be used in a more aggressive or confrontational situation. "Call out" can also be used to indicate vocalizing loudly and assertively. Furthermore, "holler out" and "bellow out" may be used in situations where the speaker is particularly passionate or emotionally charged. Ultimately, the appropriate synonym will depend on the context and tone of the communication.