Traumatic SCI involves demyelination in addition to loss of cells and neural circuits. In this case, remyelination may be predicted to induce important but limited return of function such as recovery of bladder and bowel control, improved breathing, or improved movement of a limb. We are conduction a human trial with transplantation of porcine neural cells in humans with SCI for the purpose of remyelination and for pain control. These are Phase I clinical trials designed to test safety only. These are early days, but we are all putting our shoulders to the wheel in helping make restoration of function a reality.
Impairment of hearing can result from multiple causes each with different mechanisms.
For some, such as sensorineural hearing loss, replacement strategies are an interesting target. I am not aware of such work in this area, but I have no doubt that such work will occur. However, I can approximate the issues for you. There are a set of standards required to demonstrate safety and efficacy in human trials and this process typically takes a decade. In the case of stem cells, more basic science work is required before attempts at clinical trials.