When someone is "singing a different song," it means that they have a completely different idea or perspective from what has been previously discussed. There are many synonyms for this phrase that can be used in different contexts. For example, you could say that someone is "going against the grain," "marching to the beat of their own drum," or "taking a different approach." Alternatively, you could describe them as "out of step," "out of sync," or simply "disagreeing." These synonyms can help you express the same concept in different ways, depending on the tone and emphasis you want to convey.