Kev and Jeff,  I wasn't really shooting for a world class thing here.  The standards don't have to be too high for what amounts to a "Music minus one" practice tool.  But I was using this to improve my skills in putting the mix together.  So I'm definitely game for trying the things you guys generously suggested. 
 
I have uploaded files to
https://files.secureserver.net/0faWeJZoXT3mzj
You should be able to access without a password.  There are two MIDI files.  The larger one has the source for vibes, strings, bass, guitar.  The other one has the source I used for the drum track.  I also included the WAV for the trombone part.
 
On the drum track, yes I was holding it back in the mix because it sounded so ordinary.  That bit came from a band-in-a-bix rendering.  I ran it into SessionDrummer3, but I never got it sounding very good.  For some reason, I couldn't get the kick drum to speak much at all.  I think BIAB must have some strangeness in the velocity on the kick.  I hadn't dig into that yet.  It is passable for my current purposes, but I'd like to learn how to make that sound better.
 
I'd be happy to record real trombone parts.  I could do euphonium or tuba as well (or cimbasso if you need a tuba-range part that cuts through like a knife.)

Have you hit this with compression and limiting yet? I'd compress first then limit. Should work great, as long as you get the adjustments right for all the instrumentation. Everything needs their own space and levels have to be right. Good luck.
 
Peace and Blessings, John

theguitarplayer
Have you hit this with compression and limiting yet?

I did put the LP-64 multiband compressor on the master bus.  I am a "yellow belt" regarding compression.  I have not developed any kind of a comfort zone for using compression effectively.  That LP-64, with its default settings has immediately improved the sound of every project where I have tried it.  I think I made some minor tweaks to the LP-64 settings.
 
I did not use a limiter.  My max gain is under 0dB without it.  Do you have any advice about using limiters on the master bus?  I take it that the normal practice is to put that last in the chain.  But how aggressive are you with limiting?  Do you intentionally push the peaks a little past 0dB and then let the limiter chop the peakiest of the peaks?

Making a little progress.  It is slow going because I'm taking lots of side trips, learning many things that aren't directly applicable to this song, but are definitely part of the overall learning curve.  As Edison famously said, "I have not failed. I've just found 10,000 ways that won't work.""
Here is the latest: http://bordogni.info/wp-content/uploads/2013/08/V17-T-Sonar-Ver4.mp3
 
Summary of changes:

1. I switched to a vibes sample with harder mallets.  I think that is probably more what you guys had in mind.  And it makes a big difference.  I was able to actually drop the vibes in the mix a little.  Now I think the guitar and vibes are very distinct.  I am thinking I have them separated too far in the stereo mix.  I put the vibes about 80% left and the guitar about 80% right just to try to hear the separation.  I don't think that is necessary anymore, and the separation sounds unnatural to me.  Any opinions about that?
2. I shifted the strings up about a 5th (inverting the chords) to get them above the vibes, guitar, and trombone.  That seems like a normal range for violins.  It is well within their comfortable range.
3. I think I went to a different bass that is maybe using a pick, or at least has a little more edge.  I added some transient processing to try to get that to bite without being louder.
4. The drums still suck, but I did work on the bass drum a little, bringing the velocities into a more consistent range and adding more downbeats, but it sounds like this jazz quartet had to settle for a polka drummer this time 'round.
5. I discovered an error/oversight.  In my Aria synth (vibes, bass, guitar, and strings), the reverb was on by default.  That was a problem from double reverb, plus it was adding the verb material into the bass track.  I killed all that and it cleaned up nicely.  All my reverb is at the track level within SONAR.  I have none on the bass, drums, or vibes.  I have a little on the guitar and trombone and a lot on the strings because they were too choppy after I switched them to short bow strokes.
6. I added a limiter on the master bus and pushed up the volume to where the limiter only trips a little.

I'm thinking the accompaniment is decent now.  Definitely a big improvement from where I started.  I need to go back in and track the trombone volume to match the comp better, but I may just do another take on that first.

I listened twice and I'm thinking all you really need to do is drop the volume back a bit on the trombone. You can hear everything else but because the trombone is so far up front it is drowning everything else out. Once you drop the trombone back a bit then check all the other track volumes and adjust where necessary so everything can be clearly heard on an even keel. It does sound much better with all the changes you have done so far. Very nice piece when you get it all fixed and balanced out.
 
Peace and Blessings, John

The overall volume of the track is fluctuating a lot.

I have advise?
 
Ask less? "Listen more..."
 
http://forum.cakewalk.com/Posts/78203?forumid=27

They asked for input/advice? 
 
I have advice?
Ask less? "Listen more..."
 
http://forum.cakewalk.com/Posts/78203?forumid=27
 
If you click the link, you'll see?
 
Yeaup... Actually most people do exactly what this poster does? They post material, they request feedback, however, they provide "none" on anything that anyone else does? (PLEASE people? keep your female dog comments to yourself? It's a legitimate observation, and that is all it is.) Please click the link above for an example of what I'm talking about.
 
Unfortunately, "People feed it?" And will continue to do so. I'm not commenting on the music? "I didn't listen..." I would have loved to have listened? And I would have done so "gladly?" And I would have provided honest feedback... However? The poster did not give me the option to either listen, or comment honestly based on their history... And my principals. 
 
It may have been excellent? (Their song?)
 
However, I'll never know... Because I won't feed it. If everyone posts and nobody listens? Well, you do the math... Just my opinion, others have their own? There's may be correct? Mine may be... But I'm allowed to state mine without animus.

Jeff's advice up stairs a bit was what I would suggest.... solo everything one at a time and get them sounding like they are supposed to sound..... not muffled but crisp and bright.
 
I generally start mixing with 2 instruments...... bass and drums. In the music I write, they are the foundation upon which everything else is built. As a matter of fact, they should sound so good that a singer could jump in and there would really be no need for anything else. When I get there, or close, I start to add the other things like guitar, piano, and more..... but after I solo them to get the EQ right so they are sounding like I'm in the room with them.
 
Starting with the drums and bass comes from the old band sound check days.... drums were always first, then we added the other stuff, and could turn the drums down as needed... if that was needed. Same thing I do in the studio.... I want to hear the drums as part of the mix and not an afterthought. I did not hear the drums very well even in the last post #14... but the style of music probably doesn't lend well to prominent drums...
 
anyway, my 2 cents

ULTRABRA
The overall volume of the track is fluctuating a lot.

As I noted above, the purpose of this project is to produce "music minus one" accompaniments for the Bordogni vocalises.  These are musical exercises, and one of the things they stress is developing control over a wide range of dynamics.  The fluctuations in the volume are to follow the dynamics marked in the music.  I realize this isn't as "musical" or natural sounding as people are accustomed to hearing.  It is a trade-off in order to make the tool effective for its primary purpose.