Generally, a person who possesses technical scientific or other knowledge that is not possessed by laymen (the judge or jury), who can apply that expertise to the evidence, and reach a conclusion.
A good expert demystifies the technology used to analyze the evidence, and helps the jury understand the evidence.
In this case, the sound experts, Nakasone, Doddington, French, Wayman, Owen and Reich were held out as qualified to evaluate the recorded scream and come to a conclusion. The defense experts said the sample was too sample, and screams can't be assigned, scientifically, to people. They used their expertise to explain the limits of the technology.
I understand that concerning their field of expertise (voice anylization in your example).
But when it comes to discerning whether or not a person is telling the truth and whether their statements are supported by the facts.....who would be a more qualified “expert” than a veteran police detective who’s job it is to do just that?