vanceen
Thanks kev999. The problem isn't no sound, it's not transport. Play (neither the space bar nor the button) doesn't do anything...

 
Having no playback doesn't necessarily rule out my suggestion about the "None" bus output. The audio engine won't start if there is nothing routed to the audio interface.
 
By the way, you haven't mentioned whether the audio engine is engaged or not. I presume that it's not.

kev999,
 
Thanks, didn't know that about no transport with no output selected. Still, that's not what was going on here.
 
There was no indication that the audio engine was not engaged, i.e. no pop-up message.

vanceen
...There was no indication that the audio engine was not engaged, i.e. no pop-up message.

 
I believe that the popup message you are thinking of is the one that appears when the audio engine suddenly cuts out. If it's not engaged in the first place then you won't see any message.
 
Look at the Transport module of the Control Bar. The symbol in the lower left corner is the audio engine indicator. Is it on or off?

I'm not having the problem anymore, so I can't check. Sounds likely, though. Good suggestion, thanks.

kev999,
 
After switching back to my standard 48/36 setup, I had the transport failure again with the same project.
 
You are correct, the Audio Engine is off. Clicking on it does nothing. I didn't find any solution apart from changing my default settings in Preferences back to 96/32, closing SONAR and restarting.
 
The RME UFX is a great piece of equipment but it doesn't seem to like sampling frequency changes. The Fireface 800 I had previously was similar. I guess that's at the heart of it, but I still don't know why the transport / audio engine off problem started happening with this project, as I've been working on it for more than a year.