Keep it down is a common phrase often used to ask someone to lower their voice or tone down their behavior. However, there are several synonyms that can be effectively used in its place. You may consider using phrases like "lower the volume," "hush your voice," "keep quiet," "whisper," or "be quiet." Alternatively, you may opt for more direct phrases such as "stop making noise," "shut up," or "zip it." The choice of the appropriate synonym depends on the context and the relationship between the speaker and the recipient of the message. Regardless of the synonym used, the goal is to achieve a quieter and more peaceful environment.