Well, no, there's actually plenty of ways to do this entirely in SONAR.
The first and best way is to do it non-destructively using a clip based envelope. Change the track Edit Filter from Clips to Clip Automation -> Gain and you'll see a red line appear on all of your clips in the track. Drag up or down or add points on the clip you want to adjust. Then change the Edit Filter back to Clips. Done.
The second way is destructive, where you select the part of the waveform you want to affect, then go to Process -> Apply Effect -> Gain...
The third way is to put a clip based effect on the clip. Split up your track so that the portion you want to affect is its own clip. Right clip that clip, choose Insert Effect -> Audio Effects -> and put in any plugin that has a gain or output control and adjust to taste. This is the best way if you want to add any compression or dynamics controls to a particular clip, like you were wanting to do. (You can also add it in the effects bin of the entire track, but I'm assuming you just want to have it work on a single clip by your post).
Finally, you have track automation in general where you can basically mix the entire track. It's usually a good idea to get your balance right first with one of the previous methods, and then use track automation to basically mix the track itself, but it certainly can be used to adjust a portion of a track too. Change the track Edit Filter from Clips to Automation -> Volume and you have a line appear across the entire track. Add points around the part you want to change and drag the line up and down over that section. Change back to Clips when you're done if you like.
For any of the clip or effect based things, if you want to make it permanent (which really isn't necessary in most cases and actually limits any changes you can do later), you right-click the clip and choose Bounce To Clip and it gets mixed down to a new clip.
Audition is a fantastic tool, but SONAR really has most of what it can do baked in.