I have just rummaged around in the registry to add Adobe Audition to my Utilities menu so I can edit audio in Audition on the fly. It works great but for one crucial issue. Am I right in supposing that Sonar will always send the entire original file to the external editor regardless of the edits you've already made in Sonar? For example let's say I recorded 16 bars of guitar, and in Sonar I chop out 4 bars from the middle. I then highlight the left clip of the two remaining clips and send it to Audition. However Audition receives the original 16 bar file, unless I bounce the clip first. 
 
Is there any kind of registry trickery I can do to make Sonar send only the selected region to Audition instead of the entire file? 

 
If you first select a region, then invoke Audition, only that region appears in Audition for edit.
 

Sharke's observation is accurate in my experience with AA4 (CS5.5). SONAR is not able to pass Audition a region (clip), as a "Region FX" would receive, but rather a wav file (since it is a wav editor). A clip is a "window" into the underlying wav file, and as sharke observed that entire wav file is passed to Audition when being invoked as a wav editor.
 
The advantage of this is that when destructive edits are completed, the original wave is overwritten and SONAR recognizes this and reimports the changed wav automatically. The downside is a discrete region must be bounced (rendered) to a new clip to send "only that clip" and the original will be overwritten unless the user remembers to 1) "save as" while in Audition and 2) manually reimport the altered clip into SONAR (highlight and hit Shift-K to find the "associated wav file").
 
For me, I use Audition in two methods. First is to remove noise and normalize raw tracks (destructive edits) prior to any work within SONAR - often the very first step after tracking. Second is when mastering, which is not a destructive edit, but rather a "save as" performed immediately after being passed the master track bounce (mixdown), then mastered from there.

I had this with SoundForge too, which is why I found the feature of limited use.
Try Ctrl+Stretching a clip (non-destructively) and then opening it in an external editor.
An external editor cannot know anything about Sonar-specific features.

Yeah I'm not to worried about it if all I have to do is bounce the selected region to create a new file, but it would be great if there were some hack to be able to send just the region.

Today no it sends the entire wave file. I was thinking about this same thing recently and I agree it would be much nicer if it bounced the selected region down automatically. Maybe I'll try and hook it up in my "free time" :) This is quite different from Melodyne ARA editor integration which is much more sophisticated and requires the editor to be able to stream the audio on its own via VST3.

Any idea why launching Soundforge Pro 11 from Splat x64 would give me a "Cannot create process" error?

 
I see now that what appeared in my Audition window was the selected portion of the underlying wav with the rest 'offscreen' to left and right.
 
Here's something I just tried.  Unlike sharke's example of sending the beginning portion of a clip, I used a section contained in the middle of a clip.
 
1. select the region of interest in Sonar then split it at both ends
2. select the new clip and invoke Audition on it
3. immediately hit Trim in Audition (proper region is already selected) to discard the extra wav on both sides
4. do what you have to do in Audition
 
[Now all we'd need would be to close Audition and have the new wav in Audition replace the new clip in Sonar and not replace the entire wav of the original clip with the new piece as it does now.]
 
5. still in Audition, Save As a new file (the "Tool copy n" gets automatically incremented)
6. close Audition, Sonar does not ask anything as it would have if I hadn't Saved As new wav within Audition
7. in Sonar, delete the split-out clip
8. import the wav saved in Audition into the space vacated by the clip (it fits perfectly)

Just a word of caution (not tried the above)... the point of a clip being a "window" into the underlying wav file and other clips being dependent on that wav is important to bear in mind. The "default" passing of a wav to a wav editor (and not doing a "save as") actually replaces the original wav file (i.e. it is GONE). This is very important to realize, since other clips may be dependent on the original file. The "GONE" is no joke... it has been overwritten by the external wav editor.
 
Without getting too involved, the "safest" route (barring Noel's comment above) is to not only split a clip but also bounce before passing it to an external wav editor. Without that bounce, the clip is not "its own discreet wav file," per se, so it runs the risk of ruining references to the rest of the original wav file in other clips in the cwp (i.e., if the original is gone/overwritten, the dependent clips will be gone too).

LJB
Any idea why launching Soundforge Pro 11 from Splat x64 would give me a "Cannot create process" error?

This is usually a permissions problem. It may be necessary to run SONAR as administrator.