Stereo Mix/Midi

well ive had my computer for about a month and ive been recording songs but whenever i go 2 record something from my computer like lets say im watching a dvd on windows and im trying to record a speech from it using cubase it doesnt seem to work and now ive noticed that i do not have a stereo mix and i was wondering if any one can tell me how to get one please.

the midi - ive been trying to make midi songs for a wile now and then turn them into instrumentals but sometimes when i go 2 play back a track - on lets say fruit loops - its programmed to Acoustic Grand Piano but then it jumps to a random instrument even though ive set it to Acoustic Grand.