Hi. I was using my mom's iMac one day and I found that the mac was on mute. so i pressed the volume buttons on the top right corner of the keyboard to unmute it. however, it did not work so i tried pressing the mute button several times. it did not work either. I then went to system preferences, sound and try to un-mute it. on 'output volume', i found a checkbox with the word 'mute' next to it. it was checked so i tried to uncheck it. However, i failed as well. is there anyway to un-mute the mac? i know that the speaker is good cuz it gives the beep sound on boot when i turn on my mac. Can anyone help me? thank you so much!