See http://www.waves.com/plugins/maxxvolume#mix-and-master-with-maxxvolume
 
MaxxVolume's claim to fame is that it can compress downward from the loudest dynamics (as all compressors do).  But it can also compress upward from the softest dynamics.  You can only do that indirectly with most compressors.  That is, if you compress the loudest dynamics, then you can bring everything up  without clipping.  But that is not exactly the same thing.  With this Waves plug, you can bring up the softest dynamics without changing how the loud ones sound.
 
Is there anything in the standard SONAR kit that can do the same thing?

There is no single tool as such but there are the Linear Phase tools, EQ, Multiband Compressor and the Adaptive Limiter.

Wookiee
There is no single tool as such but there are the Linear Phase tools, EQ, Multiband Compressor and the Adaptive Limiter.

Can any of these boost the softest material without affecting the louder bits?

I use 2 compressors or limiters to handle this and its almost always in the mastering stages.
You'll have an over head limiter to avoid overs but also a set compressor to pick up lower levels and boost them when they get below a certain point.
 
Im not sure your going to find a limiter or compressor like that in Sonar. As Wookie said, the adaptive limiter is a great tool and will get you half the way there.
I typically use the L3 and the MV2 in conjunction with one another to achieve this result.
Everyone has their own way of doing things, personally I have been at this a long time and when I find something that works, I tend to stay with the bread and butter play.
 

cparmerlee

Wookiee
There is no single tool as such but there are the Linear Phase tools, EQ, Multiband Compressor and the Adaptive Limiter.

Can any of these boost the softest material without affecting the louder bits?

Honestly I do not know, I think the Adaptive limiter will get you part of the way there but as I said there is no single tool in SONAR as the Waves tool you linked to.  Chuck made a valid suggestion. 

chuckebaby
I typically use the L3 and the MV2 in conjunction with one another to achieve this result.

It looks like MaxxVolume is a combination of those two rolled into a single plug-in.  With MaxxVolume, you can compress down from the louds, you can compress up from the softs, and you can do general leveling.  It was on sale for $29 but has gone back up to $49 today.
 
The idea of compressing up from the softest material sounded surprisingly good on the demo.  It brings in extra detail without changing the energy that the loud material imparts, so it really isn't quite the same as normal compression.  People describe that as helping to save a track where the microphone placement might not have been just right.
 
I almost bought it yesterday, but I didn't want to add yet another plug-in if it was redundant with stuff already in SPlat.  I wanted to get some opinions yesterday, but have been having trouble using the forum from my network at home. 
 
I guess I will wait for it to go on sale for $29 again.

cparmerlee

chuckebaby
I typically use the L3 and the MV2 in conjunction with one another to achieve this result.

It looks like MaxxVolume is a combination of those two rolled into a single plug-in. 

Not even close  .
 
In a dynamic sense, yes.
The L3 is a multiband which includes IDR and ACR. MV has neither.
But for 49.99 (that's what Waves is listing it at right now) it is a good deal.

It uses a compander (compression/expander) so expands the louder frequencies on and after threshold  & also keeps them from clipping. As far as I am aware there is no dedicated expander and compressor unless you combine the two prochannel settings with the PC4K S Type Expander/Gate and S Type compressor. Then follow that up with a limiter making everything Post Fader.
 
 

interpolated
It uses a compander (compression/expander) so expands the louder frequencies on and after threshold  & also keeps them from clipping.

If I understand MaxxVolume, it doesn't have an expander, but its unusual thing is that it has a "bottom-up" compressor.  If we have a track that has dynamics ranging from -2 down to -40 (or even lower I guess) you can set MaxxVolume to leave -2 to -15 alone, but bring up the -40 to -30 to hear more detail.  I don't see any way to do that with the standard SONAR tools.

Possibly by side chaining a compressor ?
I don't know to be honest, I've never looked to Sonar for these tools. I do use Sonar plug ins in my projects and quite a few of them but when it comes to dynamics and EQ in the mastering process (and mixing for that matter) I tend to lean on high end plug ins like Waves.
 
The problem with MaxxVolume is there is no multiband compression. So any of those low offending frequency's need to be dealt with first. for example just boosting -2 to -40 could potentially make hiss, low end rumble more evident.
When boosting lower levels one needs to take caution of what is down there. IE- the noise floor.
 
 
However when compressing and limiting dynamic material, such as a hard rock song that drops out and has an acoustic guitar only, we want that acoustic guitar to be perceived just as loud as the harder parts. not exactly the same but relatively in the same manner.
 
a novice mix is often spotted very quickly when the dynamics get softer and the bottom drops out (it doesn't sound as loud anymore).