How to mute the microphone on Key-Down and unmute it on Key-Up?

What I would like to have is:

1. By default the microphone should be muted
2. When CapsLock is Down the microphone should unmute
3. When CapsLock is Up the microphone should mute

(Basically convert my Skype to act like a Walkie-talkie with a push-to-talk functionality)

I found the AppleScript that can change the volume input of the microphone, so the first step was easy, but I didn't find any way to even assign the CapsLock to a script. Note that my CapsLock key is turned off and I would't like to sTART experiencing something like tHIS.

If the CapsLock is very special I wouldn't mind using another key, but I want to achieve the same functionality.

Edit: Probably it is not really possible to do it without using any application, so if there is an app that can do that, it would also work for me.

First, the AppleScript that inspects the key state and handles the mic enabling and muting:

if (modKeyDowntest()) then
  tell application "System Events" to set volume input volume 100
  repeat while modKeyDowntest()
  end repeat
  tell application "System Events" to set volume input volume 0
end if

on modKeyDowntest()
  set modKeyDown to do shell script "~/opt/checkModifierKeys shift"
  set modKeyDown to modKeyDown as integer
  set modKeyDown to modKeyDown as boolean
end modKeyDowntest

Note, that

• the script depends on the checkModifierKeys command line app (and it's path).
• inspects the state of shift instead of caps lock (other possibilities are cmd, option, control and caps lock - caps lock, however, means the status of caps lock, not the key state!)

In the second step we bind the script to a key shortcut. There are many applications that can do this (some listed in an answer in SuperUser) or as an Automator service. (C&P from Lri's answer...)

One more option is to save the script as an Automator service:

• Open Automator and choose the Service template.
• Utilities — Run Shell Script / Run AppleScript.
• Service receives [no input] in [any application].
• Add the script to the text area and save.
• Assign a shortcut in System Preferences — Keyboard — Keyboard Shortcuts — Services.

So, I assigned opt+shift+M to the script above and it enables the mic, keeping it unmuted as long as I hold shift. The mic is muted again when I release shift.

I have not used it, but Skii plugin for Skype, seems to provide what you're looking for:

Skii is a very easy to use and simple plug-in that uses the Skype API to simplify the Push-to-Talk function. The app gives the user the possibility to control the Skype Push-To-Talk function with just one button! a small utility that will definetly improve your Skype user experience but also your overall workflow.