Double check ... is there a space before the mp3 or after it, such as . mp3 or mp3 (space)
Do you have other audio players you can test it with? Do you have VLC player, try that.

There is also a very small case it's not an mp3 but someone has renamed the extension. I know on windows systems it treats files according to their extension, so if the extension is mp3 that's how it will treat it. Yet it may actually be a wmv or m4a file for example.