Perhaps I misunderstand, but if mute/unmute the mic would help, why don’t you just stop speaking for 5 seconds between the prompts? Do you want to reduce the low level noise that would create between prompts, so as to produce absolute silence? If so you could use the Noise Gate plugin.
If the prompts are the same length and you don’t want to wait before speaking the next prompt, you might be able to use the Regular interval audio splitter plugin.
If the prompts are not the same length you could try the experimental Extend Silences plugin.
Those three plugins above are Nyquist plugins. See http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#install for how to install Nyquist plugins.
If you still want to a shortcut to mute the mic, it can be done by creating a toggle input mute “service” with Automator then add a shortcut for the service: http://eddmann.com/posts/creating-a-mac-microphone-mute-keyboard-shortcut/.
Gale