(1) Get nircmd from <a href="http://www.nirsoft.net/utils/nircmd.html" rel="nofollow">http://www.nirsoft.net/utils/nircmd.html</a> (it's a zip file, no installer)
(2) Put the nircmd.exe wherever you like, e.g. into your document folder
(3) Create a new shortcut (e.g. on your desktop) by right-clicking and selecting "New...">"Shortcut". Select the nircmd.exe from step 2 as the target.
(4) Right-click the new shortcut and select "Properties"
(4) In the Properties window find the field "Target". It has the path to the nircmd.exe in it. append to the end "mutesysvolume 2 default_record" (there must be a space between exe and mute).
(5) Click into the "Shortcut Key" field and press your desired shortcut key, e.g. CTRL+ALT+M.
(6) Click ok.
Now every time you press CTRL+ALT+M your microphone* will mute or unmute. (May take a second, though.)
If you want one shortcut to mute and one to unmute, just create two shortcuts, one with "mutesysvolume 1 default_record" to mute and one with "mutesysvolume 0 default_record" to unmute.
*) Actually this mutes the default input device. Read the nircmd documentation to figure out what to do to mute other devices.
