Muting one track (but listening on headset) while recording live on second track

Gonna need some more details on what exactly is going on and what you are trying to do. Usually you'd just turn down your monitors and monitor through the headphones while you track and mute/solo whatever you don't want/want to hear. However if you are looking to create headphone mixes you may want to look at what your interface can do in that regards.

Posted before I saw the last reply. So all the sound is coming from a keyboard synth that's being direct-monitored via the Mackie? Or you have a soundcard also sending audio to the Mackie or do you record from the Mackie's Main Outs?. Sounds like you might have the soundcard set to "What U Hear" or "Stereo Mix" where you're monitoring from the output of the soundcard instead of the input, but your routing still isn't clear enough to be sure.
 
 

If I listen to the headset plugged into the mackie mixer I hear only the solo instrument.  If I plug the headset into the speakers I hear only the background track.  I'd like to hear both at the same time while recording only the solo track.
 
I hope this is more clear. 
 

I'm not using the synth -- the cords are wired THROUGH the synth into the mixer (I rarely use the synth).  Right now I have an MP3 file on track 1.  I want to mic a solo on track 2.
Make sense?

Sorry... I'm not following this routing situation at all. Ideally you want all your tracks to have their outputs going to the Master bus in Sonar (either directly or through other busses but just get them to point to the master for now) then have the master pointing to the monitor outputs on your interface or whatever is feeding your monitors. Then you should be able to hear both tracks at the same time and can adjust, mute, solo, etc.
 
So track 1 (backing track) and track 2 (live input track) > Master > Interface/Soundcard monitor out > Monitors/Headphones.
 
Sounds like you have things routed all over the place but isolated from themselves... but I'm not sure. :-/

Well that's the thing... you've got the tracks being outputted to two different places (still not sure how... I'm not familiar with the equipment you have mentioned). What you want is Sonar to output everything to ONE device that feeds you monitors. Basically you need an audio interface (ideal) or figure out how to send sound out through your computer's onboard soundcard (not ideal).
 
Otherwise I guess if you have a mixer you could send the two separate signals you currently have feeding your two separate sets of headphones to two of the mixer's channels then listen through the mixer's headphone outputs. Just make sure it's a LINE level signal, not something coming from a power amp, or you might blow something. Turn down the monitors while you are recording so you are recording the monitor output through your sax mic.
 
 

Ok, I'll need some time to work on it tonight and THANK YOU so much for your input. :)

I don't have a ton of time to spend on this, but maybe this will help.
 
Since you have a mixer at your disposal, getting in touch with how it works will help you get this all working.  as mentioned by others, you need to get everything going to one place and then disseminating the signal out from there to where it needs to go.
 
Here's what I see you doing (do all of this with the volume on everything turns WAY DOWN as there are chances for feedback until you get everything setup right):
 
1.  Connect your sound card to inputs 1 and 2 on the Mackie.
2.  Connect your monitors to the Main Outs 1/2 on the top right of the Mackie
3.  Connect your headphones to the Phones jack on the top right of the Mackie
4.  Connect the audio output of your midi keyboard to input 3/4 on the Mackie
5.  Connect the Aux 1 Out from the Mackie to the Input on your soundcard
 
To hear the mains, turn up the main outs on the Mackie (and make sure that inputs 1/2 have enough signal from your sound card)
To hear the headphones, turn up the headphones on the Mackie
To hear the keyboard, use the volume on Input 3/4 on the Mackie
To adjust the record volume of your mic, on the channel where the mic is plugged in, adjust the AUX 1 Send knob.  Do not turn up the volume to the main volume while the monitors are turned up here or you will get feedback.  Also, do not ever turn up the AUX 1 send on the channels where the sound card outputs are plugged into the Mackie.
 
See what this gets you.  Please be careful about the feedback thing and be very methodical about all of this.  Your mileage may vary as I don't know the specifics of the routing within the Mackie - so you might have to mess with things a bit to get them to work right.
 
The point here is that the Mackie then becomes the central control point for both your record and playback.
 
Hope this helps.  Gotta go - maybe someone else can chime in on this idea.

<p>Thanks, Blades, I copied this and will be working on it later.&nbsp; You wrote a lot so thank you and Beepster for taking the time to help.&nbsp; MUCH appreciated!&nbsp; MUCH!</p>