  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 normalization 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, within the next decade, a window
on string-theory's extra dimensions before their eventual compactification.
