For what it's worth - have you verified that the mp3 plays in another 
player?  You probably still shouldn't get a walkback, but you might just 
test it.  If you want to send the mp3 to me off list, I can test it in 
my Squeak too.

Richard A. O'Keefe wrote:

>I wrote that my attempt to play what I was *told* was a .MP3 file
>using the MPEGplayer failed.  One correspondent suggested that
>the problem was having an old MPEG plugin lying around.
>
>No, I didn't have any MPEG plugin at all.
>
>Another correspondent suggested that the problem was that I *didn't*
>have the MPEG plugin, and indeed I don't.  I would have expected that
>if you try to call something from a plugin that is not available,
>you would be told 'you need the XYZZY plugin for this'.
>
>So I searched for MPEG in the Swiki, found the MPEG plugin .sit file,
>downloaded it, unstuffed it, and tried again.  Same problem
>MPEGFile class>>primFileValidMPEG: aPath
>    <primitive: 'primitiveMPEG3CheckSig' module: Mpeg3Plugin'>
>failed.
>
>The mpeg3Plugin file is 379,499 bytes and has a creation date of
>31 May 2002 and a version of 'Squeak MPEG Plugin V1.3 http://www.squeak.org'.
>
>As for SqueakAmp, it flatly refused to believe that
><root disk>:Desktop folder:
>was a valid folder name.  (It is.)  Is there any reason why SqueakAmp
>doesn't let you navigate up and down the folder structure in the usual way?
>Typing entire file names is not the Macintosh Way.
>
>  
>

-- 
Jason Dufair - jase at dufair.org
http://www.dufair.org/
"My grandfather once told me that there were two kinds of people:
those who do the work and those who take the credit.
He told me to try to be in the first group;
there was much less competition."
-- Indira Gandhi