The DLC characters did have voices, but there were things to note about them.
Skarlet - A whole new voice, but seems to have less sound files than the others; she does not have unique fatality death screams.
Kenshi - New voice; at least one unique fatality death scream exists.
Rain - Seems to be using heavily modified versions of existing "pain grunt/yell" voice files; fatality death screams follow the previous note.
Freddy Krueger - Sounds like he uses heavily modified versions of existing voice files, most notably Cyrax's "burning reaction"; Freddy is the only DLC character to say "No!" when Shang Tsung's Identity Theft Fatality is performed on him. (All other characters, including Kratos, say "No!")
Kratos - Uses existing voice files from the God of War games (e.g. his intro), but he uses at least one existing (modified) MK9 voice file for his reaction when an attack lifts him off the ground.