Didn’t saw that in previous posts. The described effect quite often show up when there are storage related problems - all RAM used, SSD almost full, CPU heavily used by other process etc - but that’s easy to spot. If that’s not the case then it can be something with WoW data archives.
I tried to get this bug yesterday and could not so I was curious if it’s related to downloading prepatch as I just let the client download it few days ago.
High CPU usage could indicate something with their data files (like CASC) as they can decompress, index, compress it which is similar to operations on ZIP files - scales to all cores.
Audio problems aside of the easy to spot causes mentioned above can happen if the game can’t access the files and that’s likely in MPQ archives. The client/updater may want to change such archive or some file index blocking even read access for the game. Something they are trying to debug apparently: Fully installed WoW is downloading on background? - #9 by Mosinathus