Ok.. let me try and explain
First of all you may or may not want to record your tweaking of the synth.. that synth's audio output.. you might want to just record the movement of the parameters you're tweaking.
If I understand that problem correctly.. the problem is you're in session view.. trying to record that automation into the clip.. and unfortunately.. it's a weakness of Ableton 8 that stuff doesn't get recorded into a clip.. but it can be recorded into the arrangement view... and edited.
Also.. to help you out... the tweaking of parameters of a synth.. what the synth will be record is not midi.. but the automation data.
Now for some synths.. there's a MIDI learn.. so maybe you have some kind of controller.. you right click on the parameter you want to tweak and.. and if the synth has a midi learn it's method of assigning midi to.. err I think you might say parameter to it... then if you move the knob on your controller.. that knob will now control that parameter.
The MIDI standard is.. more complex and deep then what folks normally utilize.. but if you were to get into orchestration.. you'd want to get deeper into the MIDI standard...
Basically.. there's a whole lot of what's called "realtime continuous data" stuff.. in the MIDI standard.. that you can be used to control whatever parameter you wish.
Now.. in the standard there's actually certain things that are.. by default supposed to control certain types of parameters of a virtual (or otherwise) instrument.. but developers don't usually implement this stuff.
In any event.. you can record this kind of midi into a clip in the session view.
The good news is that Live 9 should fix this issue of recording into the clip.
Now.. you don't have to send audio through your interface to record stuff.. and you wouldn't want to do this as this will degrade the audio and introduce you to issues with latency and... just a lot of pain.
However if you look at an audio tracks ins.. you'll find that it has an input option for every channel you have in Ableton, as well as a "resample" option. The resample will record everything as it comes out of the master channel.
Now the only problem is.. I'm not 100% sure that everything I'm telling you is 100% true.. cause there's a few things I'm fuzzy on in there..
What I will say is now would be a great time to go get some training...
check out the all you can eat options at macProVideo.com - Streaming Tutorial-Videos & HD Training Downloads for Audio, Graphics, & Video Editing Software
as well as Learn Pro Tools, Logic Pro, Ableton Live and more - Training and Tutorial Videos
and also check out Tom Cosom Tom Cosm
who's really awesome... Lynda.com has some Ableton video's to.. but last I checked Lynda.com didn't have stuff that really went too deep into audio stuff.
But these sites will walk you through all the basics of music production and how to use ableton.. and a whole lot more.. they're really great resources.