playing asp audio files

I found some answers, but firefox is still a mystery:
 1/ tha .asp file contains a link to a .ra file. So the problem is with ra audio files
 2/ used outside firefox, realplay and mplayer works.
 3/ from firefox, I got the message:

       xxx.asp could not be opened because the associated helper application
       does not exist. change the application in your preferences.

    Would it be so difficult for them to say what is the application which does no
     exist? How can make a change it if I don't what to change?

     I tried to modify edit/preferences/applications, putting realplay
     as application for the 2 RA file types there, but I still got the same error.
     Can anybody explain that?

  4/ at last, I found in prefs.js a line(God knows where it comes from):
        user_pref("plugin.disable_full_page_plugin_for_types", .....audio/x-pn-realaudio......

      I removed audio/x-pn-realaudio, and now firefox opens the mplayer plugin for
      the ra files, and it played the audio files several times... but now, it stops
       just after connecting.
     Again, have anybody a possible explanation?

.asp is a file extension used for websites (like .php and .htm). It is not an audio file.

A .ra file is a Real Audio file which can be played using Real Player. (Which I don't think is present in the default repositories so you will need to use the binary installer provided by Real Media).