You don't need a mic to hear, you just need one to speak. When you log in, there is an option to 'Let me hear the voices' or 'mute all speaking'. You need to make sure that you select the one to 'hear the voices'.. or whatever it says. Just don't choose 'mute all speaking'.
That way, if you don't have a mic, you can still HEAR everything that is being said, but you just cant respond by voice. Make sense?