IBM Watson supports MP3 transcription and my testing shows it is very similar to WAV in terms of overall quality.
The benefit, of course is it greatly reduces the voicemail file size.
If MP3 is not set for the system then it will use WAV.
If other users prefer having the option of MP3 for the system but WAV for Watson transcription we could add a new variable for Watson transcription and add this new variable as a qualifier.