Ideas for soundbanks:
Game can inform by small icon next to each player if its soundbank exist or not and will be used default one instead. It maybe speaker icon with green color if soundbank exist, red icon if not (will be used default one instead), and maybe yellow if soundbank is being downloading (if game start before fished this, it use default one). When pointing on that icon by mouse, it may show soundbank name, files size, and if downloading, show its progress in %. Downloaded soundbanks may have own folder (User\DownloadedSpeech\*soundbank_name*). In game settings add option for downloading missing soundbanks from players (on/off), so when its off, it will only inform about missing soundbank, but not downloading.
Optional: Comparing soundbanks contents by file sizes for make difference if different soundbanks have same name. If same named soundbank with different files downloaded, save it with [1] prefix on end or so.
And mainly, game may look for soundbank in BOTH folders: User\Speech, and DATA\User\Speech (in this order). Oh, in User\DownloadedSpeech\ too ofc.