Sonar 8.5: Splitting a single Mixdown Track into Separate Audio Tracks - Is It Possible?

I have this audio track, where all the instruments and vocals are all in one track, and I want to split it into each individual track again. Is this even possible? The reason why I'm asking this is because I had a mishap with my machine back in June of this year, and when I restored it back to working order, the hard drive on which my projects are stored was accidentally formatted. Unfortunately the original project from which this audio mixdown came got erased with it, so now I'm just left with the mixdown track. I would rerecord the vocals, but they won't ever sound the same as the track I mixed down when I originally exported the audio. What can I do next?

Unfortunately there's no way to "unbake" a cake...the best you can hope for is that the vocals are mono and panned to center. Then you can throw one channel out of phase, sum them in mono, and (maybe) cancel the vocal. You'll also cancel anything else that's mono and panned to center, like the kick and probably the snare, so you'd need to overdub new parts for those. 

Interesting question, let me take a completely different view on this...clearly this is the poster child example for making project backups on a regular basis.  As well stated above there is no such technology as unmix or unrender when all you have is the mixed or rendered file. However, I occasionally have had a need to surgically pull out elements (mostly voice) from a rendered file that the source tracks were no longer available.  I use Izotope's RX Advanced to do some of this work.  This answer comes from a slightly different domain than from where your question is oriented, but with the proper time and skill put in, there are some significant things that can be performed when combining the filtering suggested above with the surgical abilities of RX Advanced.  Literally if this was a lost Beatle's take, and they wanted Lennon's vocal isolated, I'd take that contract and ask for a year to do it.  So the balance is related to the time to do the work versus the re-record.  I'd do the latter 90%+ of the time.
 
Best
 
-D