What is the aural equivalent of eye-witness?
The OED lists ear-witness as a valid word:
A person who testifies, or is able to testify, to something on the evidence of his own hearing.
The earliest citation is dated 1597 and the word is not listed as obsolete or even archaic, but there are no recent citations, either. Take that as you will...
I would say there is no equivalent since the word eyewitness ("a person present at an event who can describe what happened") covers the concept already. Otherwise you might also need a word for a touch witness, or a smell witness.
Divorced of its literal meaning, that is. Not every gentleman is gentle, after all.
Only witness would suffice in case of aural evidence.