I've read several posts in the past that many people think Frank only has two voices that he manipulates, and while I see their point, I find several things very interesting:
• Frank wouldn't perform any voices in the begining because he believed he couldn't.
• Frank wasn't crazy about singing because he thought he couldn't.
• He has more popular and regular characters on SS and TMS then any other performer!!!
Whether or not he can do different voices isn't the issue in my opinion as his range of personalities is very notable. In one special/interview, he was listed as being cited as the greatest puppeteer of all times. Truthfully for the most part, I think most seasoned fans can here a voice and know if it's Frank, Jim or Jerry. So I'm gonna' go the oposite end of the spectrum and say that Dave and Richard have the most extensive range and say Jim and Frank have a more limited range, only they are such brilliant performers that they can almost disguise that.