A recently proposed mechanism for large-scale structure in string cosmology—based on massless axionic seeds—is further analyzed and extended to the acoustic-peak region. Existence, structure, and height of the peaks turn out to depend crucially on the overall evolution of extra dimensions during the pre–big bang phase: Conversely, precise cosmic microwave background anisotropy data in the acoustic-peak region will provide a window on the extra dimensions from string theory before their eventual compactification.