Not-a-Ref doesn't work with sound files?

    I'm using LabVIEW 2013 32 bit on W7-64.  When I open a sound file and am writing to it, e.g. the ref is valid, the Not-a-Refnum? function returns True.  Is this a known limitation of sound files?  The same thing is true with the NI example program "Sound File to Sound Output.vi" when a sound file is opened for reading.  The VI works, but the Not-a-Refnum? is True.

DaveT