libsdl-mixer1.2 is supposed to be in the Maemo Chinook repository for maemo 4. I think I've added the repository but can't find the file.

Also, how can I see what repositories are installed on my n810?

Open application manager, press menu key and find the repositories or catalogs item in tools menu. Too lazy to start up appman right now to verify the correct menu names but it will be obvious when you dig through the menu.