The custom mirror directory parameter is used by Switchover, which MIC doesn't support. The recommended config is to store all prompts on a network location that is reliable.
It's been a long time since I've looked, but I don't think the web attendant stores the prompts on the web server - not for playback anyway. Those should also be in a network location, but I forget just how that is configured. The docs should mention where the attendant prompts folder is configured.