A modern account of the ear treats it as one organ doing one general job, hearing, and stops there. Samkhya's account of shrotra is narrower and more exact in a specific way: shrotra is defined by the single tanmatra it can register, sound, and by nothing else, and the physical ear is, in the classical model, only the gross housing for a subtler faculty that the ear alone does not explain. This matters because the whole apparatus of perception, in Samkhya, is built from a strict one-to-one correspondence between five sense organs and five subtle elements, and shrotra is the clearest case of that correspondence, since it is difficult to describe hearing as doing anything except registering sound.
Shrotra is produced, along with the other four Jnanendriyas, from the sattvic aspect of ahamkara, the same source that produces manas and the five organs of action. Sattva is associated with clarity and lightness, which the tradition takes as fitting for organs whose job is to register information cleanly rather than to act on the world, the work assigned instead to the Karmendriyas.
Classical enumerations typically place shrotra first among the five Jnanendriyas, mirroring the position of shabda, sound, as the first and subtlest of the five tanmatras, from which akasha, the ether or space element, eventually condenses. The ordering is not decorative: Samkhya's evolutionary sequence proceeds from subtle to gross, and sound is treated as the most subtle, least resistant of the five sensory qualities, present even in empty space where the other four are not.