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.