I've had this problem since Friday with my Mac Pro (March 2009) and haven't been able to solve it. On Friday my speakers suddenly stopped working. After first having confirmed that the speakers were working with another source, I tried headphones both in frontal headphone slot and with the analog line out in the back, both without success. There's a red light in the optical audio out, indicating that it's on for some reason, although I have selected different audio out source. The internal speaker works just fine, only the normal audio out doesn't give me anything. I found a thread in the MBP section similar to this problem, but solutions for MBP didn't work. Has anyone had this problem before? If so, how did you fix it? Any hint or advice would be greatly appreciated, as at the moment I don't have the time to send my Mac to repair. Thanks!