Using the tablet's browser, go to http://www.gronmayer.com/it/ and type xmms in the search box. It'll search for repositories containing any packages with xmms in the name. The repository that should come up is debfarm.free.fr; clicking the green install arrow should install that repository. (Well, it'll prompt you to save or open, choose open, then app mugger will come up, prompt you to make sure you really want to, and then it'll install the repo...) Afterwards, xmms &c. should show up in the list of all installable apps, down quite near the bottom.