Hi there
If you're looking to create an audio (.wav) CD, the last thing you want to be doing is assessing the relative levels of your tracks when they've been converted to mp3s, no matter how good MP3Gain is at doing this. There are a number of good arguments for not using mp3 files in this process, not least of which is that the mp3 files you create will not be a true representation of your original mixes; mp3 encoding is a 'lossy' process (even at 'good' quality 320kbps) and will not contain all the sonic information of the wave files you created them from.
The process of creating a CD with tracks of similar 'loudness' that you describe is actually one of the main parts of the 'mastering' process. As there are many different approaches to accomplishing this, I'd recommend that you read up a little on the subject.
To assess the relative levels of all your tracks at the most simplistic level, you could simply create a (blank) project in SONAR and import all your songs to
separate tracks. You can then 'use your ears' to judge the differences by playing the project and swapping from song to song using the Exclusive Solo feature.
For a more accurate result, you could also enable
Peak Markers and SONAR will indicate the highest level any of your tracks attains. To enable this feature, click on the Track View
Options menu and select
Meter Options > Show Track Peak Markers:
Play through all the songs in your project and the peak level (in dB) will be recorded in the individual track panes. The feature also marks the track at the point where this peak occurs:
As this feature also works on busses, you could check the level of the Master Bus in each of your individual song projects to get an idea of their highest level.
Mind you, I would strongly suggest you do some reading about the
whole Mastering process, as it involves a lot more than just 'normalising' the levels of the individual songs.