Temporary unmute

I need an answer to this as well.  On one of my class days, I must take a long lunch at work,  I do not have the opportunity to retreat to a quiet office.  Everywhere I go, there is a radio over the PA(retail).  I need a quick solution where I can mute the microphone for the majority of the class time, but if I have a question, be able to quickly unmute the microphone to ask the question, then quickly mute it again.  Someone told me about the space bar, but it did not work the last time and I fumbled around the screen looking for the mute/unmute.  The background noise in my establishment does not make it convenient to use the computer audio and microphone at work.  I wear headphones over my ears, but then cannot participate very easily.  Are there any short cut tips anyone could give me?

Tracy