Play Microphone Sound on keypress?

I know the general form for running a function (or whatever) upon a keypress, but is there any support/ workaround for sending sound through a microphone channel rather than playing it through the speakers?  For example, I want to play a sound through VOIP (Skype is an example), whenever I press (for example) the "-" key.  

If there is a documentation for this, I have not been able to find it, so any pointers would be appreciated.