Official comment
DO NOT use the MIC input for a line level signal! It's not designed for that.
I'm in the process of updating all of the guides for broadcasting, so you can either search for 3rd party info or hang tight until the new articles are posted.
And to be perfectly honest, Zoom is not the best platform for that - consider simply broadcasting rather than hosting a "conference." Use something like Twitch, YouTube, Mixlr, etc.