Never mind the MONO button(or the lack of one)
. The MUTE you need to hit is the MUTE button on the recording track itself. By muting the track, you won't hear the recorded sound slightly later than the original sound. You hear the original sound by adjusting the MIX knob on the Mbox. While that makes punch-ins tough, the way to do fixes is to learn about the playlist feature and track comping. Or you can try setting the buffer to the 128 setting(or 64 if its available) and leave the MIX knob all the way to the PLAYBACK side. Now you would not mute the recording track and you would hear sound only from PT. Only you can decide if the slight delay is okay or not.(where you record, say 3 tracks of vocal, and then make a COMPosit track out of the 3 takes). There are some good tutorial videos available between AvidTV and youtube.