Equalising many audio clips

There are a couple of things: firstly, MP3 is not the format to be doing the processing in at all. You've encoded your mix from the wav file that Audition would have started out with (even if you thought you were putting MP3 files in, you weren't; they were decoded to wav before Audition would do anything with them), and then you've re-encoded this to MP3 again, and then when it's got to Premiere it's been decoded a third time, had the levels messed with and then you want it as an MP4? I fully expect it to sound pretty bad after this.

Secondly, whilst this methodology will certainly set the peak levels to be the same, it won't necessarily have the perceived volume the same. The degree of discrepency depends mostly on the dynamic range of the materials; Audition does have a way of levelling out perceived volume, but I don't think it's likely to survive your present processing regime...

What you should do to preserve what quality you can is to export the audio as a wav file. This is uncompressed, and won't degrade - also you could volume-level it first without it getting messed up. You import the wav file into Premiere, and when you've got everything the way you want it, you then export it to MP4. This way you only do one decode of the MP3s you started with, and no more encoding of anything until the last step. The basic principle is always that if you are going to encode anything to a lossy format (MP3 and 4 are both lossy), then you should start out with the highest quality original, to minimise degredation. With MP3 files, especially if they are only 128k ones, three decodes will make them sound sort-of 'plastic' - nothing like the originals at all, and not pleasant to listen to.

As for the loudness, when you've got your individual files open, you can drop them onto the Match Loudness box (if you can't see it, you have to enable it in Audition's Window menu) where I just dropped a single file, select a processing option (try the one in the screen-grab below) and after you've run it, save the modified wav files, and those are what you use in Multitrack, and the basis of your export.