When I look at Audio.h library, I see that the 2 DACs are enabled (stereo). I don't think this library can be used to play .wav files in mono, only in stereo.
The header of a .wav file (first 44 bytes) provides informations on the number of bites per sample (e.g. 16) , sample rate (e.g. 44.1 Khz), whether it's a mono or stereo recording and the number of bytes to read. So "all" you have to do is open the .wav file, extract the above informations, transform 16-bit samples into 12-bit samples and program the DAC peripheral accordingly.
IMO, the way the PDC DMA for DACC is handled in this library is really weird.