I don't use mac but did you verify that the O/S has mic/webcam option enabled? I know in Window 10 if you disable mic/webcam other apps will not be able to use this function.

I looked at all the settings for my mac and so far everything is enabled for discord.

Once logged into Discord, I unmute the mic and then get the prompt to allow. see screenshot I then went into the settings in Discord and was able to test the mic. see screenshot