This just happened to me on a project. Any other solutions! All other projects play audio fine. 
 
My timing is set, all routings are correct. No meters moving, nothing.
 
 

I've had this happen a lot - I don't know what triggers it, and when I dig into settings everything seems to be fine. Some projects seem more susceptible than others, though I have not been able to figure out why.
 
By now you may have already tried this, but I found that closing sonar and re-opening the project solved the issue short-term. Eventually sound for some or all instruments would disappear again, but closing and re-opening would reset it. If you're on a deadline and pulling out your hair, hopefully that will at least get you through to the finish line.

Yeah I tried closing and opening it, restarting, even uninstalled and reinstalled my audio interface software/drivers.
 
This is really weird for sure.
 
I can preview samples in the Media tab in the browser and I can hear it. I can click the keys on a softsynth piano and the audio comes through.
I can arm a track for a mic input and hear when speaking into the mic.
 
Just all the audio and softsynth stuff already in the project do not play at all and the meters do not show at all either.
 
What a trip.

Another thing to try that I forgot about until I was looking at another thread about something unrelated:
 
Unplug any midi controllers, restart, see if the issue persists. There's a chance a midi controller is sending unintended signals and screwing things up for you.

Tried that. No luck.
 
I will keep trying things and let you folks know if I find a solution.
 
Thank you kindly for the input thus far.
 
 

Ok an update.
I go to an any soft synth and click the midi input and change it and then I can play the keyboard and the softsynth plays.
 
After that, when I play the project, no sound and that softsynth no longer plays again until I click on that midi input again and re-select it.
 
Also, still, the guitar parts and vocals which are only audio still do not play. Total silence and no meters when playing back the project.
 
I wonder if there is some kind of reset for the project I can do or something.
 
 
 

Well I still could not find the culprit so I made track templates of the tracks in the messed up project and set up a new project with the same tempo, opened the track templates there and copied over the clips to the new project on the tracks they belong to. It is playing back everything just fine, audio and softsynth tracks, no problems.
 
Took about 45 minutes. At least it worked.
 
Sheesh...