xmms-arts does not have a dependency on xmms nor xmms-libs. I'm not aware of a case where an external app that can use xmms plugins would benefit from xmms arts output, so I'd suggest adding a "Requires: xmms". Also, perhaps the *.la in the package could be dropped? Also #2: xmms-libs already owns %{_libdir}/xmms/Output, no need to own it in this package.
I've added a Requires: xmms-libs, since I suppose some program could use the library as a backend and want to output to aRts... I've also removed the .la file and removed ownership of the Output directory. Thanks for the report!