Unfortunately this is a rather dead site now that Sims 3 came along. I have no idea how to do that.  You might have better luck asking at Mod The Sims. At least you will have more people seeing your question.

My only guess would be to back up your neighborhood, find a sim with that voice that you don't mind losing, and try sim surgery. I'm not sure if sim surgery moves the voice of the donor to the patient or not, but this is part of the reason that you backed up the neighborhood. I'm pretty sure that voice was one of the things that got left alone. (I never really paid attention to the voices. My sims always ended up with the same ones.)

If you end up having to copy the entire appearance of the correct sim to the sim with the voice, you can use the sim blender to more safely change all the details of the sim to make them match the one you wanted, then age the original to Elder and give him a birthday with Inge's cake,or age him down and let the sim have a son who looks like him.  ( I don't need to tell you to be absolutely certain that you are looking at the right age group...)