Can't? Well, pretty much any voice that has a level of gravely or raspiness to it - I don't know how Jim was able to do voices like Rowlf or Dr. Teeth, or how Frank did Grover or Cookie Monster without losing their voices altogether. I can do Grover to a degree, but it strains my throat after only a sentence or too. If I have to do such a voice, what I usually is rather than force the gravel from my throat, I kind of talk inwardly, if that makes sense - it creates an effect similar to raspiness without irritating my throat so much, but it only works to a degree with certain voices and not so much with others (like the aforementioned Muppets).