I have songs recorded in Sonar X1 Producer that were done using a PreSonus 16.4.2 board. I now have a Behringer XR18 and can record and playback new songs, but I am unable to playback any of those older songs recorded using the PreSonus board. The error message I get is 'Unable to open audio playback device'. How do I 'resurrect' my old tracks.

After you open the old projects make sure that your master buss is outputting to the new XR18. As long as its listed in your devices in preferences it should use it.
 
If perchance you do not have a master buss now is a good time to add one and point all your tracks to it.
 
Comment; I have a ton of old projects that were created with different audio interfaces. I have never run across this issue.

Hey John, thanks for the quick response! Whenever I click on the master buss, it shows a list of the XR18 channels. I choose channel 17 (aux input) for playback and the list goes away. I click again and there is no checkmark by channel 17, prompting me to choose again (and again, etc). I click the play button and get the same message. The error message also says that the audio format may not be supported. Does this have something to do with sample rate of original recordings?

You have to choose a XR18 port, a stereo out, for example.  If you choose an aux port, you'll have to go to that aux port and set a XR18 out so that your board can play it...

Atsuko, having trouble figuring that out. Can you explain further?
 

Ricky,
if you already have a Master bus, you should locate its drop-down list for output and click the small arrow to show all available outputs. Select the output that your speakers or headphones are connected to. After that, you change all the instruments tracks and set their outputs to the master bus.
 
Example: Guitar track -> output to Master
              Master -> output XR18 stereo (output 1 & 2)
 
See if this documentation can help you:
https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Mixing.14.html#1750199
 
 

I initially had this issue.  With My Presonus FirePod, I recorded some songs as 88/24.  They would not open.
The XR 18 is 44/16 or 48/24 only.  I use 48/24.  As long as I had tracks that were 48/24 on the old system, I could get them to work.
 
For tracks recorded at 88/24, I just imported the WAV files.  Sonar did the conversion on import.  Yes, you loose all the settings and plugins, but at least you can use the files you have tracked.  I typically do Audio Only, so I don't know how to get the MIDI to transfer.