Thanks for the info.

If this is happening on both machines, looks like the files are no good. There are spam files that are deliberately misleading.

Checking the size can help: Most mp3's as you probably know, are almost 3mb's or more.

If you are pretty sure that the file is legit (can you try it on a third machine?), then the OS'es on your machines need to set the application you want. In the Finder on OSX, you click on the file and set the default program using "Get Info."
In OS 9, the Control Panel File Exchange mapps the file to the app. (btw--10.2.3 is up to 10.2.6, so you could run Software Update--might work on the OS 9 machine too)

iTunes has a preference to set itself as the app for Internet files, and just in case you haven't tried it, drag the mp3 onto iTunes.

Cheers