Apparently these are the additional cast members for the 'complete' version of the WWZ audiobook, which fleshes out the previous (abridged) version.

Simon Pegg as Grover Carlson
Paul Sorvino as Fernando Oliveira
Henry Rollins returns as T. Sean Collins
Ade M’Cormack as Jacob Nyathi

Common as Darnell Hackworth
Brian Tee as Hyungchol Choi & Michael Choi
Jeri Ryan as Maria Zhuganova
Masi Oka as Kondo Tatsumi
Frank Darabont as Roy Elliot Bruce Boxleitner Gavin Blaire
Nathan Fillion as Stanley MacDonald
Rene Auberjonois as Emil Renard
F. Murray Abraham as Sergei Ryzhkov
Alfred Molina as Terry Knox
Nicki Clyne as Sharon
Denise Crosby as Mary Jo Miller
Parminder Nagra as Barati Palshigar
Martin Scorcese as “Breck”
Scott David Ogden Stiers as Bohdan Taras Kondratiuk
Kal Penn as Sardar Khan

More info is here: Max Brooks Zombie World