Put a file link to the MP3 in the Comments - it's HTML so something like
<a href="file:///whatever.mp3/">MP3 File</a>
should do the trick. You'd then play it from Book Details - click on it and it should invoke whatever program you have associated with MP3's - WMP, WinAmp, Squawkbox etc - Note Calibre needs to have triple slash in file & folder URI's.
If you make a long text Custom column for it then it wont get clobbered if you happen to do a metadata comments download from Amazon etc.
Below are a couple of screen shots of the metadata editor and how the Book Details might be arranged to accommodate audio books. One could imagine movies and TV miniseries being handled similarly.
The Related Materials tables you see in the screen shots, is how I'm starting to deal with the "Supplementary Files" issue, its come up several times in this forum in just the few months I've been using Calibre. I will write something up in the next few days - its actually worked out easier than I thought.
If Audio Books were an issue for me then I'd probably utilize another table with details like Reader, duration etc. Some book reading audios are in 'episodes' so one might need separate entries for each episode, a tabular approach would be useful in that circumstance. you'd want that for TV miniseries too.