Audio engine won't start - SOLVED

On my laptop (for temporary use while travelling), I have several Sonar projects in which the audio engine will not start.  On playback, only the midi tracks' meters move.  Audio and synth tracks show no activity.
 
This seems to have happened because I have edited each of these projects while the audio interface (Line6 GX) was disconnected.  Obviously I was not able to playback during this period, but I didn't really need to and it was inconvenient to set up the whole kit at the time.  But now that it is re-connected, the audio engine button does not respond for these particular projects and remains off, while all other projects that I did not edit while the GX was disconnected work fine.
 
Any ideas?

Make sure your recording and playback timing master is listed as your sound card in the options > audio . advanced menu.
When You do stuff without your audio interface in sonar, it defaults back to no card. so you may have to redefine some settings, like your playback and Recording Timing masters

A speedy response!
 
None of the audio settings seem to have changed and other projects playback ok.

Lol, thanks, I would switch to wdm and profile the sound card again or delete your aud.ini file and restart sonar. either one will do

Thanks for these suggestions.
 
Normally Line6 devices require ASIO mode for full functionality.  However, this is not an issue at the moment, as I am not intending to do any recording while using the laptop, so I will switch to WDM.
 
Having switched to WDM and run the profiler, no change.  I also tried deleting aud.ini and restarting.  Still no change.

I've seen this with free-ware plugs (even supposedly good ones). I usually have to delete plugins in safe mode to get the project to play again. Odd thing is that the project usually plays fine as I'm adding all the plugins (free and purchased ones) is when I start the same project a day or two later that it won't start the audio engine again. I sometimes think that staying with mostly waves plugs, or mostly uad plugs keeps the project more stable, since at their core they are probably calling common dll code.

Every now and then I'll do an edit or tweak a plugin setting on a large project that's running fine, and audio will no longer start playing. At this point I usually save the project to a modified name, and restart Sonar.

Just a thought did you click on the audio engine on button?

Yes.  There is no response when clicking on this button

In the past, I have only ever seen the audio engine unable to start in an overloaded project where playback stops too.  But in the current situation, absence of audio engine does not prevent playback.

By the way, latency is currently set high - around 100ms.