I've been playing Rage: Campaign Edition and enjoy it very much. No problems until recently.

Suddenly, late in the game under Nightmare play mode, the various characters went mute. I don't believe muties are responsible. The voices just disappeared entirely. The music soundtrack and sound effects are fine, they remain unchanged; but the character voices are gone.

What is it and how do I fix it? Thanks!

Hm first of all - check your audio settings in-game