Samkhya splits the eleven organs into two groups with sharply different jobs: five Jnanendriyas that receive information and five Karmendriyas that act on the world, with manas coordinating between them. Shrotra belongs firmly to the receiving half, and the classical texts are careful to keep that division clean, an organ of hearing is never assigned any active function, and vak, the organ of speech, which does involve sound production, is classed among the Karmendriyas rather than treated as a variant of shrotra, because producing sound and registering sound are, in this system, different kinds of operation belonging to different tattvas.
This division reflects a broader structural feature of the twenty-five tattvas: Samkhya consistently separates the passive, informational side of experience from the active, causal side, even where a modern account might collapse them into one faculty. The five Jnanendriyas, shrotra among them, are purely receptive; they contribute no motion, no effect on external objects, only the delivery of a specific kind of data to manas for synthesis.
That receptive character is also what makes shrotra's scope so narrow. An organ built only to receive does not need the flexibility a general-purpose faculty would; it needs only to be tuned to its one tanmatra. Shrotra's entire operation is exhausted by shabda, sound, which is why Samkhya can maintain, without strain, that the five Jnanendriyas and the five tanmatras stand in a fixed one-to-one correspondence running through the whole of its evolutionary sequence.