What was the reason they hired William E. Martin to voice Shredder in season 8?
 

I find it odd since Townsend Coleman and Jim Cummings who voiced Shredder were both voicing characters in "Get Shredder". Niether of them voiced him for some reason. I'm guessing they wanted someone with a more menacing voice for Shredder in the darker episodes.

I still prefer James Avery, but I wonder if his voice would had fit in the red sky era. Some people say William E. Martin sounded creepier and more menacing. What is your opinion, guys?

While I never read an outright reason it can be speculated that James Avery was overbooked with Fresh Prince commitments and other projects and had to drop out of the role. I belief Jim Cummings and Townsend Coleman were in house substitutes until a regular voice actor could be found and that was William E. Martin who had a good voice for Shredder. It sounds like a scruffier,slightly sick verion of Avery.

^ What he said.

exactly. except, i wouldnt necessarily say "overbooked", as much as "felt he didn't need to continue doing voice work anymore".

He was already gone for most of Season 7, so they probably figured it was time to drop him.

I'd love to hear Mr. Avery make an appearance on Rob Paulsen's Talkin' Toons podcast. Maybe they can shed some light on the subject.

I'm curious how many people auditioned for Shredder at that point, given that the show was in decline and the fact that Shredder only appeared in seven more episodes for the rest of the series. Jim and Townsend were probably too light for a Red Sky Shredder, given that Bill's Shredder sounds very deep and serious, though lacks the edge that James Avery's Shredder contains. I'm not sure if CBS got involved to say "no comedic Shredder" as well.