The first teaser for the new Michael Bay produced, Jonathan Liebesman directed TMNT feature debuted last week to mixed reactions, but without much in the way of speaking from anybody but William Fichtner (Erich Sachs/The Shredder.) Apparently, that's because they aren't done casting voices for the entirely-CGI main characters.
But they're getting closer. Deadline Hollywood reports that onetime Jackass leader Johnny Knoxville (most recently seen under several pounds of Academy Award-nominated makeup in Jackass Presents: Bad Grandpa) will voice the Turtle's field-leader Leonardo; while Monk star Tony Shalhoub (who worked with Bay in Pain & Gain) has been tapped for Splinter, their rat sensei and surrogate father.
The move is somewhat unexpected, as the characters were created using motion-capture and facial-capture technology; meaning that Knoxville and Shalhoub will effectively be "dubbing" characters whose vocals and corresponding mouth movements have already been recorded and animated.