The five Tanmatras are the tattvas most likely to be misread by analogy with a modern physical vocabulary, and shabda is the clearest case: it is tempting to hear "sound" and picture vibrating air reaching an eardrum, but shabda in Samkhya's scheme precedes both. It is a subtle quality produced directly from ahamkara's tamasic aspect, prior to and more basic than akasha, the element that will eventually condense to carry it as an actual physical medium, and prior to any ear capable of hearing it.
This ordering, quality before element, element before organ-object relation, is the structural signature of Samkhya's whole account of the tanmatras. Shabda does not wait for akasha to exist and then attach itself to it as a property; akasha comes to exist, in the evolutionary sequence, precisely because shabda already does, condensing out of it. The tanmatra is prior in the order of explanation, not merely prior in time, which is part of why the classical texts describe grossening as tanmatras becoming elements rather than elements acquiring qualities.
Shabda's place as the first tanmatra, matching akasha's place as the first and subtlest gross element and shrotra's place as the first Jnanendriya, sets the pattern the remaining four tanmatras follow: each new tanmatra in the sequence is one degree denser than the one before it, and each corresponding element inherits every quality already established plus one new one of its own.