On occasion no sound from Sonar

do you use a control surface ?
its possible your hardware fader is pegged off.
 
more info will help here. like for example..
when you have no sound, do you see meters moving ?
 
tried headphones when this happens ?

Chucke and BF:
Thanks for the responses!! To answer your questions:
 
I do, in fact, see the meters moving, but don't hear any sound. 
I don't use a controller surface.
Headphones don't have sound either.
 
No Asio4all.  I use the drivers for a Roland Quad capture interface.
I do have my interface set as the default playback device for windows.
The projects are of all sorts.  Sometimes only midi.  Sometimes only audio.  Sometimes a combination.
 
I'll definitely try toggling the audio engine, that seems like an easy fix.
 
Again, thanks for your replies.

Does solo come on for any muted track?

Sometimes I have to twist the output fader on my UCX interface to start it moving signal after a power recycle.

In general though, I don't have this problem.

Make sure you have the latest QuadCapture driver, and to Dan's point about USB, don't let Power Management turn off USB ports and don't let your PC sleep/hibernate if this is occuring after that.
 
Also, try ensuring that 'Always Stream Audio Through Effects' is enabled in preferences, and maybe add an empty, muted audio track with Input Echo enabled. When I've had issues like this with certain interfaces/drivers in the past, having input monitoring enabled helped keep the audio engine and ports alive when the transport was stopped.

I've been having a similar problem. For me, it's not when I first open the project, but at seemingly random times while I'm working.
 
Sometimes, the playback would continue fine, but suddenly there would be no input. As with jude77, if I close the project and reopen it, I'm back in business. I always disable the wireless network adapter before starting, because of audio problems in the past. One time I noticed that the Realtek adapter, which had been disabled, was back on, probably from a Windows 10 update. Since I disabled it again I haven't experienced that. I have no idea whether or not that is what actually made the difference.
 
However, recently on one project, which is all MIDI so far, it's the playback that will stop functioning. I don't recall it stopping while playing, but sometimes after I've stopped it and started again. This time, the meters show activity for playback and input as well, just no sound. Again, closing and reopening returns things to normal. I've been spending time with two other projects lately, and haven't been experiencing this with either of them.
 
I've been assuming it has something to do with my soundcard. It's an M-Audio Audiophile 2496. It has no Windows 10 driver. I'm using the most recent driver. However, I only seem to have problems with Sonar. Everything else (SoundForge, internet, etc.) works fine.

Gotta say, I have seen this happen occasionally, too.

Bristol_Jonesey
When you say you see the meters moving, which meters are moving?
 
Track > Bus > Master > Mains
 
If there's activity on all of them then the problem is external to SONAR, most probably with your interface or driver.

For me, it's at least the track and master. It didn't occur to me check the main. Next time it happens I'll look at that.
 
I've been assuming that it's external to SONAR, but it's interesting that it hasn't happened anywhere else. I also wonder why closing and reopening the project makes it work again, at least temporarily. It's not even necessary to close and reopen SONAR, just the project. Also, so far this particular problem (meter activity with no sound) only seems to be happening with one project.

Same question:
 
When you say you see the meters moving, which meters are moving?
 
Track > Bus > Master > Mains
 
If there's activity on all of them then the problem is external to SONAR, most probably with your interface or driver.

Which strongly suggests it's your interface > monitoring.
 
Check your interface and monitors for loose connections - sometimes rebooting can sort of "bridge" a slightly loose connection - it happens with the surround sound on my cinema system all the time. Jack the volume up or switch it on/off and the sound in the rear speakers comes back.
 
Check your USB cable - swap it out if possible with a known to work, good quality one.
 
Also see if there's any setting on your interface or the driver software which might be contributing.

Many thanks everybody.  Will do!!

The more I think about it, the more I think it's my interface.  I have a Roland Quad-Capture that has been a bit of a disaster.  The 48v power died on it after I had it about a year, and the USB power cuts out completely on occasion.  I've swapped the Roland for an old Line 6 UX1 I have.  We'll see how that does.
 
My sincere thanks to EVERYONE who answered my distress call.

TimV:
I'll be glad to.  I should know in a day or two.

I only just read the thread but already at the top I thought ... seen that - that's the damn Roland driver again ...