I've made 28 demo tracks for our band's repertoire, all nicely mixed and mastered in Ozone7. Each demo was a separate Sonar project.  In order to give them out on one CD I've tried hard to make them all sound similar in terms of loudness just using my ears, but I end up going back to successive projects to tweak their loudness via their Ozone7 controls and re-exporting. I'm in danger of getting into a  loop of continually 'shortening each table leg'.
Any quick and easy tips?  All I can think of is to import all 28 mastered WAVs into one Sonar project so that toggling between them is quicker and easier ears-wise, but then any that I change will need to be re-exported, is that ok quality wise?

I don't know what "mastering" you did but "mastering" is actually a large part of the problem you're describing. If you're mastering for an album and the songs need to sound like they belong together you should definitely have them in the same project. Bringing their volumes and sound images together is part of mastering for an album.

Hopefully someone with Ozone 7 can chime in. I only did the trial, but the standalone version seemed like it had a layout to support album level matching. Not sure on that though.

If you have Adobe Audition, there is a feature called "match volume" which can be run as a batch routine. This will yield "RMS normalization" and has a few other options. Other wave editors may have similar, not sure, but Audition does for certain.

Ozone 7 may already have such in standalone mode (first option), but if this is a one-time thing, the trial of Adobe is fully functional (or used to be - check first).

Skyline_UK
 
 All I can think of is to import all 28 mastered WAVs into one Sonar project so that toggling between them is quicker and easier ears-wise, but then any that I change will need to be re-exported, is that ok quality wise?

 
That's what I've done in the past, it's quite easy to line them all up on one track and jump from one to another to check relative volumes.

I monitor the loudness of songs as I mix them using an European Broadcasting Union (EBU) meter. This has a thing that measures loudness over time. I tend to target -14 dB for most music, but sometimes go for -12.
 
When I measure LANDR masters, the lightest level of compression is usually around -13.2 dB loudness averaged over the song.
 
In any case, I imagine I'm bending something fragile to squish it into a box. I want to bend it as little as necessary to fit in the box I have for it.
 
To this end, I balance the various bending I do. I try to limit very little. The Concrete limiter shows how many dB it has limited at the max if you keep it open during playback. This is handy. Usually if it goes over 2 dB, I look to notch the spots with a volume envelope or perhaps add some lighter compression to the track that's causing the limiting.
 
So...
Playback and watch EBU meter. Set limiting sufficient to get to desired level on EBU meter (like -14).
 
If limiting is too much or becomes noticeable, increase compression only until you get the average loudness you seek with the limiting restrictions you set for yourself.
 
Now, one song, you may have to increase compression and limiting and another song you can back off compression and limiting. This may be counter-intuitive. In other words, Loud songs may need less compression and quiet ones may need more. It depends on the musicians playing the songs, but a loud distorted guitar often needs very little compression. The distortion IS the compression.
 
When a band has every song played loudly where the loudness is steady, that's when I consider mastering it to -13 or -12 dB. There are few peaks that need to be limited to get those extra few dB.
 
I've heard a lot of mixes mastered much louder than -12 dB, but I never do that. These are like the LANDR loud mixes. It just doesn't do it for me.
 
Now, when I finish with a show, all my tracks are roughly the same loudness. I might say this track needs a little more bass, or I want to hear more vocal there. Loudness, however, is constant.
 
It is true that one band mastered to -14 I can listen to at a nice high volume on my stereo where another also mastered to -14 will cause me to turn it down. Again, this might indicate that the first of the two could be upped to -12 dB.

Skyline_UK
 
 All I can think of is to import all 28 mastered WAVs into one Sonar project so that toggling between them is quicker and easier ears-wise, but then any that I change will need to be re-exported, is that ok quality wise?

I do this, put Ozone on the master and watch the meters, then use exclusive solo to toggle between songs.  The meters are not enough to get the songs to appear the same volume.  Use exclusive solo and your ears to fine tune and you can get them to match nicely.  FWIW Studio One is fantastic for this. You can create a mastering project like you are attempting very easily and volume matching is much easier.

Good meters. good ears, good dynamics/eq tools and a good plan it the only way to properly match songs.

That said, a Bluegrass number should not really be expected to match a Deathmetal track.

28 song demo?

Is this for a new member to learn songs or something?

If it really is a demo, I would pick your best 4 and match them up, much easier.

It is a hard problem, esp. over entire projects that gone on for a long time.  Even if you get your songs to look the same (on the timeline and on a 'scope etc.) they can still sound softer or louder, esp. going from rock to ballad and such.  So finagle your meters etc. and get the stereo masters as close as possible, then listen.  Where science stops, craft and art begin.
 
If you shoot for -.3 dB when mixing and song mastering you won't go over when converting that master to mp3.  That also gives you some room when project mastering and matching the relative volumes of songs.  If you song master to -.1, you have painted yourself into a sonic box and either you won't be able to fix the overall project or your mastering engineer will shoot you since he can't either.

Some excellent suggestions here.  You could go with a lot of different stuff here but I think the simplest thing would be a Limiter like Boost11 or the Concrete Limiter.  Make the output the same for all of the songs i.e. -0.3.
 
rafone

Yes indeed I agree with using tools like Boost 11 and Concrete Limiter.  I produce a lot of different styles of music and I always use Boost 11 and the concrete Limiter in the master bus they even things out and achieve a constant output volume.
You might also try using the Channel Tools in the master bus you can boost the mid and side channels a little to give the tracks some clarity.  If your tracks were recorded at consistent levels and your mixes are good your mastering should be consistent with the use of these tools.
 
Blackie James