I belong to a Community Chorus and have been placing Practice Tracks on a Website for members to better learn the parts.  I am wondering if there is  an application  (perhaps using Sonar Platinum) that will individually accentuate the Soprano, Alto, Tenor and Bass parts so that they are dominant and stand out from the other parts.

Seems obvious, just lower the level of the background parts. 
Are these audio singing parts or a piano guide track? 
I have a friend who makes practice CD's for the local Choir and those are done on a digital piano with her singing the required parts. She uses a Tascam Handy recorder live, no multi tracking. 

to add to what Johnny said, panning might also help.
Baritone in the left, Soprano in the right.

I never tried exporting a song like this but maybe using dim solo might work, just solo the track you want to emphasize and put on the dim and the soloed track is louder. 

If the Tracks were separated into Multiple Tracks, yes, those ideas would be an obvious thing to do, however, the tracks are taken as recordings of full-up chorus ensembles and there is no way that I know of to separate the multiple voices.  I've tried using  various techniques including band pass filtering but it doesn't work as many of the parts overlap on the frequency spectrum. Melodyne might work but it is very very time  consuming as far as I can tell and a lot less than perfect! 

Oh, that I'm afraid is a different kettle of fish. As you already found out trying to break a stereo/ mono file into multiple parts is near to impossible. Each part is a complex spectrum of frequencies and overtones. 
 

Ya this isn't 2050. I thought you had multi track recordings.
We are still in the early stages of digital audio (20 years).
maybe in 20 more years you can dissect a full mix. Melodyne can do some of that but it takes a lot of time.
Over lapping vocals though, I don't think so.

Thanks for your inputs.  Maybe in 20 years (hopefully less) we will be able to separate out the vocals by assessing  and separating the resonance and timbre of the voicing.  Each voice is unique ... at least that is what the security people attempt to portray to their consumers. I may be way off base here; but, do you agree that if one's spoken word can be differentiated from another's, then surely one's musical voicing can be also differentiated?  Just a thought!  I wonder if Melodyne's developers have played around with that premise!

If they are all mixed together in stereo one trick to isolate different things is use the Channel Tool. You can isolate things that are in the Middle or in the Sides. Depending on how things are panned you can bring up certain things and get rid of others, unless they are all panned in the middle. This in addition to EQ could do what you want.
Also like Chuckebaby said, Melodyne can be used to split the notes in to polyphonic blobs that you can then delete the parts you don't want. The Melodyne trick will work better if there is just the choir voices without piano. It can be done though. Aside from that, you can record yourself singing along each time singing the part you wish to accentuate. Make separate mixes for each part with you singing the appropriate part.