Ahamkara's productivity divides along guna lines, and shabda belongs to the half most readers find less intuitive: the tamasic aspect, bhutadi ahamkara, which produces all five tanmatras, while the sattvic aspect produces the eleven organs. Tamas is associated with heaviness, inertia and obstruction, qualities that seem an odd fit for something as apparently immaterial as sound, but the classical logic is about density and objecthood rather than about audible heaviness: the tanmatras are what will become the physical, resistant, external world, and that destiny is tamasic in Samkhya's technical sense regardless of how ethereal any individual tanmatra seems compared to solid earth.
Shabda, as the first tanmatra to be produced, is the least tamasic of the five in relative terms, the lightest and least resistant, which is exactly why akasha, its corresponding element, is described as offering no obstruction and simply providing the space in which the other four elements, each carrying one more accumulated quality, come to exist. The chain from shabda to prithvi is a chain of increasing tamasic density, one tanmatra and one element at a time.
This tamasic sourcing distinguishes the tanmatras sharply from the organs, sourced from ahamkara's sattvic aspect, and the distinction is not cosmetic: it explains why the organs are described as clear, receptive instruments while the tanmatras and the elements they become are described as the resistant, external objects those instruments perceive. Perceiver and perceived, in Samkhya's account, both trace to ahamkara, but by two different roads.