It sounds like you're using a Simple Instrument Track. For some reason, Cakewalk has seen fit to exclude the interleave button from the audio part of the track. If you want to change it to mono, you have to right click on the track and select "split instrument track." You will find the interleave button on the audio track it creates. You can then right click on one of the split tracks and select "create instrument track." Or better still, just leave them split.