I routinely use Samplitude and Band-in-a-box for purposes where SONAR is less intuitive.  I found from use of previous versions of SONAR, that although Samplitude and BB can run simultaeneously and copperatively use the ROLAND sound card, SONAR can/does not.  I would quit SAMP/BB and then SONAR would run normally and use the sound card.  Now, with the upgrade to X3, when I load SONAR it says it can not find an audio interface and opeing Preferences, the ROLAND card is not even listed.  I have to reboot and run SONAR first for SONAR to identify and see the ROLAND interface.  Is there a fix or is this a new "bug"?

is this possibly a 44/48 thing? sonar can't reset an audio adapter to a different sample rate. if either of the other projects are different than what the sonar proj is, it won't be able to play.

There is a setting in your SONAR preferences that allows the sharing of the audio drivers.  That would be the first thing I would check.

You know, after seeing this thread again, I seem to remember similar issues even with other DAW hosts.  Meaning, I think it may be a Samplitude issue where it sometimes doesn't let go of the audio drivers.  Then it requires a reboot.