Fruit Ninja uses ogg files for sound and it seems that N900 offical libsdl-mixer lacks support for these: http://talk.maemo.org/archive/index.php/t-81792.html. But there seems to be libsdl-mixer1.2 version 1.2.6-5+0m5+ogg+mp3 in extras(?) repository. Can you get this version installed and test fruit ninja with that?