I encountered this as well and thought it was an unforgivable glitch. As it turns out, it has to do with our audio settings. The Wii U has some weird home theater issues. I have surround sound speakers, but when I run the audio test in the Settings menu, it doesn't work perfectly. I left it on the Surround settings despite not hearing some of the tones, and that's when I noticed some cutscenes in Epic Mickey and COD were missing voices.
When I switched to Stereo, everything was fine. I have no idea why, but it's probably the same for you.