there are a couple of possibilities here :
1) You can go ahead and install the xmms-mpg123 without taking
the unmet dependencies into account (rpm --nodeps option see man rpm) ... this is not recommended
2) Uninstall the currently installed xmms package and install the required xmms package (rpm -e) ... then go ahead with the xmms-mpg123 ...
3) use apt-get for rpm (see http://apt-rpm.tuxfamily.org
this (debian origin) tool greatly facilitates problems as you described ... apt-rpm installs packages WITH all the dependencies taken into account.
Hope this helps you a bit.