Quote from the manual:

<b>"MCI_command(str)</b> This functions sends the command string to the Windows multimedia system using the Media Control Interface (MCI). It returns the return string. You can use this to control all sorts of multimedia devices. See the Windows documentation for information in how to use this command. For example MCI_command('play cdaudio from 1') plays a cd (after you have correctly initialized it using other commands). This function is only for advanced use! "

It explains pretty much everything that has to do with MCI commands and Game Maker – but the content is missing. Because the MCI commands themselves is not to be found anywhere in the manual. Neither are they specific to Game Maker but, as it is pointed out in the manual, it’s a part of Windows Media Control Interface. I happened to find a very good help file containing all of the functions, which I uploaded here:

http://gmtutorials.com/files/MCISTR.HLP

I haven’t really looked it through as much I would like and I can’t say exactly which functions are included, but basically you should be able to do all sorts of stuff like recording from microphone using MCI commands.