I just tested the procedure above on my Model 12 -- I was able to import a WAV file from the MUSIC folder on the SD card into a Song. It was a stereo export from a song I created with the Model 12It took the stereo track and split it onto 5 & 6, or I could have picked one of the stereo channels instead. VLC reports the file that works is PCM S24 LE, stereo, 48kHz, 32-bits per sample.
When I try to import a WAV file I created in Reaper and put on the SD card however, it says "File Error" when I pick the file. I can't play that WAV file on the Model 12 in SD Card Play mode either -- I get the same error -- so I presume it's something about the formatting I rendered the WAV file as. This file that doesn't work is 32 bits float LE, stereo, 48kHz, 32-bits per sample -- so it must be a problem with the codec in my case.