Mode switches from CW to Phone -- huh ?

CW Morse
 

Occasionally during (CW) contests, I press F4 to send my call and a window pops up saying it can't find a WAV file, as if I'm in phone mode.  The K3S still says CW, and the title bar on the submission window says "7039.49 CW Elecraft K3 VFO A", as expected.  If I enter a call and hit tab, the signal report comes up as 59, not the expected 599. 
Restarting the N1MM+ and/or cycling the K3 power doesn't seem to get things back on track.  But something unknown happens and I'm back in CW mode (until later).
Operator error, no doubt.  What might I be doing wrong?
TIA   Don   K2BIO

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