not playing media files

[Ajaya Pradhan]

Dear Friends,
          I yesterday loaded my computer with Ubuntu 7.04.But the problem is that it is playing non of the media files. It is requiring some kind of codec and displaying that the required applications are not installed.Can some one please suggest me how to play media files on it.Also please suggest me how to connect to the internet through a proxy server in it,I tried but I cann't.
                                                 Regards 
                                                 Ajaya 
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