mettelus
I am glad you chimed in, since I do not understand the structure of these files. I have only tried this test once, and Windows Media Player "failed" on three different files. Are you implying that Windows Media Player reads Audition files properly for you?
MP3 files (like all lossy-encoded file formats, as well as lossless FLAC) consist of a sequence of discrete data blocks. With CBR, all blocks are encoded identically and are therefore all the same size (as determined by the quality setting) regardless of content. VBR allows each block to be encoded at a different quality, so that quality can be adjusted up or down (within limits you specify) as needed based on the complexity of the audio within a given block.
It might seem that cheap storage and broadband connections have made VBR obsolete, but that wouldn't be entirely true all the time. Sometimes, you do care about file size. For example, I use VBR to get files under the 10MB limit imposed on a free SoundClick account. Other free file hosting sites such as SoundCloud don't limit individual files but instead give you a fixed disk quota for all files. And whenever file size does matter, VBR will generally yield higher fidelity than CBR at medium to low quality settings.
Of course, if you're hosting files yourself or just copying them to your MP3 player, then just make them all 320 kb/s CBR and forget about it.
I just did a test to make sure I wasn't blowin' smoke about Audition-encoded files...I loaded up a half-dozen VBR MP3s that I'd encoded with Audition and played them with Windows Media Player. It correctly reported the running time for each one.