Umwelt is an accessible editor and viewer for interactive, multimodal data representations that include visualization, textual structures, and sonification. Existing systems for creating multimodal representations prioritize the visual modality, asking blind and partially sighted people to create non-visual outputs by first specifying a visualization. In contrast, Umwelt uses a shared data abstraction that enables users to output all modalities from a common specification, and rapidly switch data mappings between different representations.