tried it but it didn't work. i've sent your explanation to a friend of mine with a newer version of sonar, so he can try it out. thanks a lot:-)
 
paul

I'm not sure now, but I assume the MIDI tracks should all be on separate MIDI channels in order to be bounced on separate tracks (??). The OP doesn't tell if that is the case. I'm almost sure I've in the past bounced to separate tracks that way and choosing "Tracks" as the bounce source.

We don't even know if it's soft synths he's trying to bounce or some other midi generated audio.
 
If it's soft synths, just freeze them - it takes a couple of seconds to do a single synth!

Good point Jonesey.
Paul, which synth are you using?
What you are describing would be the expected behavior for TTS-1 with all channels set to output 1.
I suspect your problem stems from a configuration issue.
Tell us what you are working with, and please include as much detail as you can.

I keep 8.5 installed on this old beater I use for internet so I just fired it up. I loaded a midi multi track song and inserted the TTs-1 using the multi output mono output option. This creates 8 tracks of tts-1 called a folder I think. 
The TTs-1 has 4 stereo outs and if you pan tracks you can create 8 mono mixes. Or 6 mono and one stereo etc. 
The Bounce to track feature in 8.5 only seems to have 1 track as the output option just as the op is stating. So this results in a one track stereo mix only. SOmeone could possibly test a new version of Sonar to see if this has changed, I will later but it might take all day to reply. 
 
But freezing the TTs-1 resulted in 8 audio tracks in the tts-1 track folder. So that's the fastest way to do this for sure. If you are using more than one soft synth you will have to perform the freeze for each synth. Please do not complain that that's too slow for you, you'll get no sympathy from this old geezer who is amazed at how fast things happen now a day's!! 

i always use separate channels for my midi tracks. but according to a friend of mine, i need a soundcard with multiple outputs (or softsynth) to bounce/record multiple midi tracks into multiple audio tracks in one single operation.

That's not entirely accurate, but there may be some truth to it, depending on what you are working with.
Again, what synth are you using?

i'm not using any synth, just the sounds from my all-in-one interface/soundbox. that may be a part of the problem, since it doesn't have multiple outputs.

Ok, we are slowly getting somewhere. Yes, that is the problem. You cannot bounce from the syth on the sound card(Microsoft Wavetable Synth?)
You are using a synth, but it's not obvious if you are not familiar with it. MIDI on its own cannot create sound. It is just instructions being sent to a synth of some kind.
What is the output of your MIDI track(s) set to?
Multiple outputs on your soundcard are not the issue. Proper routing and understanding how this works will get you going. 
We can help with that.
Please give us specifics on your system, especially the sound card.
 
ps: standing by with Sonar 8 open on my older machine. Setup is not the same as newer versions.

i'm using a roland sd-50, it'a combinated soundcard/interface with integrated sounds. my midi tracks outputs are set to this device.