I think what it boils down to is this:
you are trying to install a conflicting version of mplayer on a system that has older major packages used to compile and install and such, I use red hat 9 now and used to swear by 7.3 and the only reason I changed was because I wanted newer packages.
however; I have installed mplayer successfully in red hat 7.3, 8 and 9. I the key to getting mplayer to work in red hat 8 in your case is to get an mplayer version that is a compatible. If you take a look at this site:
if that doesn't work, try installing an older mplayer file, you can always upgrade, if memory serves me correctly, i was using mplayer 0.90 (slightly different) from here:
those rpms were compiled to work with rh7.3 so they should work in 8 since they are based on the design (as I said earlier, rh9 is very different at the core than 8 and its predecessors)
as is mentioned on my guide, you should find virtually all the files you need. again, if you are trying to use a newer mplayer file, that is probably why it isn't working.
on another note, depending on how much of a pita it would be to back up any of your current files, I would heartily recommend an upgrade to red hat 9 - and i would say a fresh install is the healthiest way to do it as you will have extraneous red hat 8 files left over if you just upgrade.
if you are into having the latest software, it is completely worth it. also, i was not a fan of red hat 8 at all, in fact, i hated it. I am not sure if you have used other red hat versions, but imho, 8 was the worst, so going with 9 should be a dramatic improvement. it's up to you though.
so as i see it, here are your options (all are quite feasible and each has its own merit):
1 - follow my guide and use the red hat 8 psyche files to install mplayer
2 - install red hat 9 and use the shrike files.
3 - use atrpm or apt-get/synaptic to get the files.
lots of options, feel free to ask if you need any clarification or assistance, i would be glad to help.