In an undertone is a phrase used to describe someone speaking quietly or secretly. There are several synonyms that can be used in place of this phrase, including quietly, sotto voce, hushed, mumbled, muttered, whispered, and low. Each of these synonyms has its own unique connotation and usage, but they all convey the same idea of speaking in a subdued or secretive manner. Whether you use one of these synonyms or stick with the original phrase, the important thing is to communicate your message clearly and effectively while maintaining an appropriate level of discretion or confidentiality.