If you mean that the ALT text for the audio file disappears after you upload it, that is definitely a bug, although intermittent - whether it is known I'm not sure.
@feditips mentioned it happened to them at least twice and it bit me with audio, as well as with video on more than one occasion, including earlier today.
Unfortunately I can't figure out under what conditions it happens - it has always been in the web client for me, but sometimes it works, and sometimes it doesn't...