Just came here for this same issue. Bizarre. Hitting that global Solo button up top did fix it but it ended up happening like 3 or 4 times in the process of working on this song. 

Are you running the latest release, and is the Aux track echo enabled? I'm pretty sure the original issue with Global Solo/Unsolo was fixed. Is everything else audible, and just an Aux track is silent? If not something else is likely soloed - invisibly or otherwise. In that case you might need to go through all tracks and buses, toggling solo on/off, starting with other Aux tracks.

just shooting in the dark: you haven't incidentally caused a loop with aux routing? Sonar might just disable it for safety (would not explain why it works in solo all mode, though) ... maybe check your sends ...
 
does it happen in other (simpler) projects or just in one particular one?

brundlefly
Are you running the latest release, and is the Aux track echo enabled? I'm pretty sure the original issue with Global Solo/Unsolo was fixed. Is everything else audible, and just an Aux track is silent? If not something else is likely soloed - invisibly or otherwise. In that case you might need to go through all tracks and buses, toggling solo on/off, starting with other Aux tracks.

I think you are right regarding latest release. I updated sonar for every update, have not experienced this again.