For the moment, the only way to get bliss mixing as it should is
volumio side :
the classic way… it mount your folder Musique and map it to /mnt/NAS/Musique in volumio
BUT
LMS side :
meaning one level up comparing to music folder configured in volumio
Of couse if you just want to use LMS and not BLISS, you could also put /mnt/NAS/Musique LMS side as proposed by Wheaten, it will work