Re: yamaha md8 multitrack
OK, you will need a stereo cable (cinch on one side, jack on the other). Connect the cinch on the audio out from your soundcard, and the jacks to channel one and two on the MD8.
you should 'arm' the two tracks on your MD8, so they are ready for recording; play your backing track from your computer > you should see signal coming in on track one and two, adjust levels so you get a nice, clean signal. Hit record on the MD8, and start playback from the computer right after that... Bingo!
All the best,