I have a Mac mini that seems to have an audio output problem. I have a known working cable (tested on another computer) plugged into the headphone jack. If I slide the balance control all the way to the left or right, I get full volume. If I put the balance control in the middle position, sound volume drops by about 90%.
On the RCA side of the wire where it plugs into the stereo, I can pull the left channel and get full volume on right and vice versa. When both jacks are plugged in, sound volume drops 90%.
I also reproduced the same problem using another cable.
The system worked properly a few days ago as wired. I have rebooted since this started.