I had this as well, and posted about it here 2 days ago.
It just happened again to me. Last time I was using duty finder to get into Baycroft, but didn't realize my chocobo companion takes me out of the duty registration. This time when I was signing up for Titan's dungeon, I had my chocobo out and it asked me to put it away before registering. About 30 seconds after I got into a Fate and noticed my sound was gone just like 2 days ago.

For the debs: connection maybe between FATEs, Chocobo companions, and Duty Finder.....?