Scene 6 In this scene we hear the Varsouviana. What is it and why is it important?

When the audience hears the polka, the Varsouviana, from the dance hall, which was playing during the scene she remembers. We are now inside her head, and the heightened unreality of the play begins to take hold. The auditory hallucination represents her guilt and obsession, and her inability to escape the past. But we hear it too, and this shared hallucination implicates us in the disintegration of Blanche's reality.