I don't know if this is an issue with the SONAR upgrade or me going to Win10.
 
If I leave a project sitting idle after playing it briefly, the audio engine does not play anything even though the meters are moving. I reset the icon for the audio engine and it works again. 
 
Anyone else having this issue?

I've had a similar symptom ever since moving to Win 10. All audio goes silent, not just SONAR. It doesn't happen often, perhaps once a week or less, but it's most likely to occur after the machine has been idle or asleep. I didn't have this issue prior to W10. I've been blaming it on the interface or its driver. I do not use the same interface as you, but like yours it is a Firewire connection.
 
I have a simple workaround. When it happens, I change my ASIO buffer size. Doesn't matter what I change it to, the point is to force the interface and/or driver to re-initialize. Sound always comes back immediately and I don't need to restart any applications.

thanks for replying - I just updated my signature, I am using the USB drivers. It is 100% everytime I stop playback and let it sit idle. It was fine before Win10. Ugh...

It is definitely Win10. Reverted back to Win8 and no issue with the audio engine sitting idle.

I've been seeing this occasionally with the RME UFX. It seems like after the system has suspended and resumed the driver won't emit audio. I close and restart SONAR and it plays again. It actually happened to me today. 
You could try contacting RME about it. I don't think SONAR is in the equation since our meters are showing activity and audio is streaming as normal.

I have not seen this.  However I can't recall leaving my audio system on for lengthly periods of time at idle. My system is set up with audio and MIDI on when I turn on my overall audio and MIDI system. its all on a single switch. The computer is independent of this.  
 
I do know that Windows 10 has power modes that are not ideal for audio. Be sure you set up your Windows 10 to not go to sleep or cut off USB after an idle time. Win 10 is meant to run on tablets and laptops as well as desktops. Right now I have my desktop set to never go to sleep. Also be sure you disable core parking. Further because Windows is always updating you may need to check these settings from time to time. 
 
Most settings are in the settings/system but there are settings you will need to goto in devices in control panel. 
 
I'm not sure this will solve the problem but it can't hurt to try. 
 
 

When I see the issue, I only leave it for about 3-5 mins. The audio engine needs to be reset every time in Win10.
 
I will checkout RME support Noel. As Dave mentioned, it happens to him after moving to Win10 too.

Just talked to the Synthax support. They have zero reports of this issue. 
 
I updated the drivers and firmware to the Feb release so everything is up to date and I am still having the issue.
 
Is there anything from a SONAR pov that I can do to help them understand the issue? If I can't get this worked out, I am going back to win8 again.

additionally, just had this happen. After the audio engine has been active for 30 mins, the audio is getting distorted now. Playback is horribly distorted. It is as if the buffers are getting hammered or something. The ASIO buffers are set to 128 which has zero issues in Win8. 
 
Resetting the audio engine resolves that too. The meters in the TotalMix are working fine. RME doesn't think it is a UFX problem.
 
This is a definite Win10 problem though. Now SONAR or RME - how am I gonna try that out? Again, works fine in Win8. 

just tried the power settings to performance in Win10. Deleted the AUD.ini, but didn't see any reprofiling. Audio still distorts and then fails to play. The meters still work in SONAR, but TotalMix is not showing any audio activity.