A screenshot might help. This is a very strange issue, one that has probably never occurred before, so it's hard to guess what it could be.
Under Options->Audio->Output Mode Settings, do you have the correct sound card chosen? Not an imaginary device or even a disabled or internal spdif connection?
What audio device are you using, and could you post the driver version?
Just by chance, are you using nVidia graphics, and if so, what driver version?