Just as the title suggests, Xmms has been removed the repository for ubuntu hardy heron 8.04. From what I gathered on my limited reading of forum posts, Xmms was too unstable and not well maintained, thus it wasnt selected for inclusion in the standard ubuntu distro. I have heard that Audacious is the replacement program for Xmms like functionality. I just installed it today and will test it out.
For those still looking for Xmms, hit this link, and download the .DEB to install.