So far I seem to have made every mistake there is to make, so when I discovered this little gem I was delighted. The POI loader will ignore .WAV files if they are not in the "correct" format. I finally discovered one format that does work with my 3790T is :
bit rate 705kbps
Audio sample size 16 bit
Audio sample rate 44k hz
Audio fromat PCM
These parameters appear in the files "properties".
If you are using Audacity to create the files, go into EDIT > PREFERENCES > FILE FORMATS and use WAV (Microsoft 16 bit PCM). Then go into EDIT > PREFERENCES > QUALITY > and set the default sample rate to 44100 Hz. Shut down and restart Audacity.
I hope this solves problems for somebody.