Rather difficult to answer with no info on your source material or the version of MEP you are using.
A first step in any event would be to go to 'File' >> 'Settings' >> 'Program...' ([Y] key shortcut) and on the Audio/Video tab check that the option 'Extract sound from videos' is ticked.
Tracks are tracks -- they can contain any objects -- video, audio, still images or text etc. As to 'optional tracks', you can set the number available for individual projects under 'File' >> 'Settings' >> 'Movie...' ( [E] key shortcut), up to 99 in some versions! Note you can enter any valid number rather than pick from the list.
To further add to what Carl wrote: if you are using the most recent versions of MEP or VPX, by default I believe, the audio track is "contained" within the video track on the timeline. In order to visually separate them you need to press Ctrl+H and then you may need to Create Waveform.