This can be related to the way the game plays back sound through the Xbox 360 itself. If the Xbox 360 is set to Surround Sound in the settings, but the device it's plugged in to does not support surround sound, the voices will not be heard and many or some sounds will not play correctly.
Please follow these steps in order to set the Xbox 360 to Stereo sound:
1. Start the Xbox 360 console without a disc in the disc tray.
2. Go to Settings and select System.
3. Select Console Settings, and then select Audio.
4. Select Digital Output, and then select Digital Stereo.
If the sound plays correctly now in the game, then you may need to review your television or speaker settings / configuration before trying to enable surround sound again.