In RH9 (at least that I know about) there is no native support for mpg123. Some issue about patents I believe.
Try this link for the rpm to enable support:
http://havardk.xmms.org/dist/xmms-1.2.7-rh8-rh9-rpm/
I installed the rpm. I know that mpeg3 is working, I haven't installed codecs yet so I haven't tested mpeg 1&2.
I hope this helps.