The players.mapping file is located in Code: /opt/mediabox/mediaplayer/players.mapping.maemo5 You can copy it to Code: /home/user/.mediabox/players.mapping and edit it there. Any entries in ~/.mediabox/players.mapping override the original settings. But if MediaBox detects Maemo5 correctly, then the GStreamer backend is the only one that gets loaded. If the main window has a close button on the top-right then MediaBox detected Maemo5. If not, then we have a bug to find and fix. Theoretically the mplayer backend might work on Maemo5 but is untested and not loaded. The osso backend cannot be used on Maemo5 since there is no osso-media-server.
/opt/mediabox/mediaplayer/players.mapping.maemo5
/home/user/.mediabox/players.mapping