Hearing impaired software goes back a ways. See Section 508 in Patent and Trademarks Document, a large field.
I have no intention of searching such a large field, but cursorily I found material back to 1994.
A search is necessary at the beginning of the patent process; it is not an easy process- that is why firms with research divisions have a patent attorney on contract.
Moreover, it must be shown that the proposed process differs substantially from patents on file.

Not pertinent to the thread, but the Institute of Biomedical Engineering, Taiwan, is a leader in the field of computer software for the hearing impaired. (I have only one working ear, hence I did some reading on the subject a while back. The solution for me was simple since all I needed was a hearing aid that delivered both channels of stereo to one ear).