Everyone is Tara Strong, unless otherwise stated.
Hannelore's mom: Kate Mulgrew, in a parody of Janeway in "Living Witness"
Claire and Clinton: Sharon Mann
Marten and Mortician's Ex: Michael Cera "I picture them with the same voice. They have similar smiles."
Faye: Debby Ryan "when disney is done with her and she becomes a has-been with an addiction problem"
Marten's mom: Cher
Hannelore: Jessica Boone "using the voice she used in Puni Puni Poemy"
Momo: Kyary Pamyu Pamyu "learning the lines phonetically" in her first body. New body: Ellen Mclain.
Winslow: auto-tuned Diego Mestanza.
Tai: If we could use dead people, Selena Quintanilla Perez. Since we can't, Penny Johnson Jerald.
Marigold: Belinda Carlisle
The mortician Steve dated for a while: Caitlin Doughty
Faye's sister: Annie Lennox
Dora: Erin Grey Van Oosbree, who played Yumi in the Puffy Amiyumi show
Pintsize: Whoever played the robot counterpart to Tuck in "My Life As A Teenage Robot: Escape From Cluster Prime"
Elliot: Patrick Stuart.
This list will be updated approximately whenever I care.