OK, I got the manual. Good news and bad news. The bad news is the MR-8 *can't* import WAV. files. It doesn't do that. The good news is- I think this is possible. but it's going to be a pain in the arse. I presume you can play the backing tracks through the headphone jack on your computer soundcard. If you can, use a Y-cable with a stereo headphone jack (3.5mm miniplug) at one end and 2 1/4" plugs at the other end (like guitar cables). Plug them into the mic/line ins on the MR-8 and record them on tracks 1 and 2. Then, record the vocal over them on track 3. The tracks from the computer will probably sound pretty bad, but that doesn't matter. They are "scratch tracks", which means we are going to erase them anyway. You can record the vocal as a stereo track on 3/4, or just record to one track and then create "master tracks" on 7 and 8. When you create the master tracks on 7 and 8, *don't* record the stuff from the computer onto the master tracks. All you want is a stereo version of the vocal. Then follow the manual, and convert that master track to a WAV. file.
Then follow the manual, and export those WAV. files to the song folder in your computer. The problem then will be that the tracks will not be in synch, time-wise. You will have to use the editing tools in the computer software to manually move the vocal track around until they *are* in synch. As I said, this will be a PIA. Then you can mix it however you want. In the future, it will be easier to record all the tracks and mix them down on the MR-8, and then export them as a stereo WAV. file. If you really have to do the mixing in the computer, you have to create master tracks on 7/8, convert to WAV. files, 2 at a time, then erase them and do it again, until you have all 8 tracks in the computer. At least then, they'll be synched up automagically.
The unfortunate reality is- you have the wrong machine to do the job you want to do. When money becomes available, consider either a machine that is intended as a computer interface, such as (only 2 tracks at a time):
http://www.8thstreet.com/product.asp?ProductCode=60669&Category=Audio_Interfaces
or a portable recorder that is *also* a computer interface, such as (4 tracks, but only 2 at a time):
http://www.8thstreet.com/product.asp?ProductCode=32943&Category=Recorders
or better (16 tracks, 8 at a time):
http://www.8thstreet.com/product.asp?ProductCode=60335&Category=Recorders
Good luck. I know what a pain that whole process is, because I have done it with an old obsolete digital recorder. Your main problem is that the MR-8 isn't made to talk to a computer, just to dump finished stereo files into the computer to burn them to CD. The 2 Zoom units I referenced above can act as standalone recorders, or as computer interfaces, and were actually built to do what you are trying to do, and much more. They can also import WAV. files, unlike the Fostex, which really is an obsolete unit.-Richie