"Unable to open audio playback device" message out of nowhere!

Sometimes I do something and realise it wasn't a clever thing to do and am not shocked that I have to go through the ritual of making Sonar recognise my Propellerhead Balance - and other times it's a WTF moment.
I can't offer a solution (other than don't try opening another form of audio player/YouTube clip when Sonar is running) but I want you to have the comfort of knowing you aren't the only one.

Grum.

I am still digging.

If you haven't done so, I recommend you configure Windows so that it plays 'No Sounds'.  I think having Windows try to play sounds will sometimes cause issues.

For me, I distinctly recall a gig where I had to reboot my computer in the middle of a set, and everyone in the club got to hear a REALLY loud sound that Windows plays while rebooting (not connected to the problem, but YIKES).

Also, please make sure your Sample Rate in Sonar EXACTLY matches the Sample Rate you have specified for the audio interface.

I did find an issue with a different interface - perhaps it applies to yours as well.  It seems that at least at one point there was an issue with some M-Audio drivers where sharing sound between Windows and Sonar was not possible at the same time.  I did not find any posting of any such issue with your Onyx 1640, and am still trying to nail down what could be happening on your system.

I am wondering though if you do not have the Windows Sounds turned off and it is trying to play some alert sound or whatever, and Sonar is not allowed to share with other programs.

I dunno - please review the above and see if any of that makes sense, 

Bob Bone

Thanks guys! Robert I will try both suggestions and report back.

Not sure, but it sounds to me as if it is dropping the Mackie drivers. Make sure in the Mackie control panel all the inputs and outputs are alive, or max it (5) or whatever at inputs and outputs. When it happens I assume the drivers would be missing in Audio/Devices? Be sure to just check the Mackie drivers for ins and outs, if 8 are inputs check them, if 2 are output just check those 2 for the outputs, and I am assuming you have monitors. If you are using the PC sound drivers for palyback and the Mackie drivers for recording? This might cause them to drop out. If that is the case, turn off the Sonar Engine during playback if possible. I have 3 sound cards on my pc, and I drop drivers sometimes. Best to just use the Mackie ins and outs while in Sonar. You can run outputs through a stereo if you don't have monitors. Just run them into an auxillary channel at the stereo, this way you can stay with the Mackie drivers in Sonar. I don't know anything about Mackie, I use ASIO drivers, and seldom drop drivers if I stay with the ASIO for recording and playback. Anytime you start swapping drives in a program a conflict can occur.