Ok, I will try I hope I can help,
 
So your guitar tracks are already recorded. In your project are you ever able to hear the guitar tracks when you playback? If not, this is how I would try to solve this problem. I would create a bus in the bus pane named "master" while using the console view i would route the output of that named "master" bus to whatever your main output would be of your M audio fast track pro.  
now I assume all your guitar tracks are in you project so once again using the console view i would route each of the guitar tracks  to your "master" bus. you should be able to hear play back of your guitar tracks. if not then there is something else going on.
 
Ok now this is where I am unsure of what your are doing with midi tracks. same concept applies however. i assume your midi routing is correct because you say you can record and hear midi though your m audio fast track pro.  in your audio tracks that the midi is routed to, set the output to those audio tracks to "master" and theoretically you should be able to hear your midi and your guitar tracks,  if not then I am sorry I was of no help ;),
 
you should be able to adjust the volumes to your liking and then you can export your project to WAV, or what ever format you need. 

Sounds like you really need to come to grips with Sonar basics. For that reason I recommend you do the tutorials that are found in the help file and the Sonar Reference Manual

The method that is most similar to a multi-track tape being mixed onto a two-track master is this:
• Record all your tracks.
• Make sure they are all routed to master.
• Instead of being routed directly to master, tracks can be routed to various buses and those buses can be routed to master.
• Set the faders and pan where you want them, or use automation to move the faders and panning.
• Select All if you want everything in your mix.  Mute or deselect tracks you don't want in your mix.
• Choose File | Export and your mix will become a wav file on your computer (like the master tape on a 2-track machine). It may take a little reading to understand the Export options.  I always check Include Live Input if I have some synths that are programmed with MIDI but not recorded.
 
Some of the other responses above are saying you can put the whole mix onto a stereo track in Sonar (Bounce) instead of creating a wav file on your computer.  This would be like bouncing down to two available tracks on a multi-track machine instead of mixing to another tape machine.
 
I know the tape comparison may be archaic and may not help; but an analog console with a multi-track tape machine is kind of a good way to visualize what can be confusing in the virtual world.
 
When I was first using Sonar for audio and not just MIDI programming, I made a very short and simple recording of a couple of instruments and messed around until I figured out how to export.
 
TAPE          SONAR
--------------------------
BOUNCE    BOUNCE
MIX            EXPORT

ericphilo
no, sorry -     I have audio tracks with my guitar already done.  I was unclear.  Trying to get those onto the audio track I've created for putting the whole thing (synth and audio tracks) into, which then becomes MP3, etc.

Here's where you are going wrong.
 
You don't NEED to create a track, Sonar does it for you, either by bouncing or Exporting.
 
If you bounce, the mixed down track stays within Sonar.
If you export it ends up outside Sonar.

Hey, I'm the idiot who used to open wave lab, set it to record "what you hear" and play the song in Sonar while Wave Lab recorded in real time. I did this for 2 years before some one on the forum mentioned "export audio" and the light bulb came on. 
OP- it take time to get your head around the way digital audio workstations are put together. 
 
Audio interface audio input= Audio track - master bus- Audio interface output
Pre recorded Audio= Audio track - master bus- Audio interface output
Midi input= Midi track-Soft synth-audio output - master buss- Audio interface output
Pre recorded MIDI= Midi track-Soft synth-audio output - master buss- Audio interface output
 
Audio interface output = to monitors 
Audio interface output can also be the source of an "Audio Export" found in the file menu. 
You can export as a Wave file or an MP3 if you have the proper encoder installed. 
 
 
 

Thanks guys.  Very helpful all, especially Pollo832. I've picked up a few things...first, the tutorials are a darned good idea.  I always just searched.  Second it appears that master bus is just a bus that I name "Master".  Geez.  Also did not know Sonar creates track.  Other great stuff; thanks for being helpful.  Next stop tutorials :)

Stick with it Eric - it WILL all make sense!