I posted this in another thread, but I'll post it here as well.
I was able to get mp3 support going with the Helix Engine by doing the following:
1) yum install compat-libstdc++-33
2) Download RealPlayer 10 for Linux from here: http://www.real.com/linux/
(Download RPM Package Link)
3) rpm -Uvh RealPlayer10GOLD.rpm
4) Open Amarok. Open the "Settings/Configure Amarok" section, then select "Engine"
5) Alter the Helix engine locations to the following:
Core directory: /usr/local/RealPlayer/common
Plugins directory: /usr/local/RealPlayer/plugins
Codecs directory: /usr/local/RealPlayer/codecs
After making these changes and restarting Amarok, mp3 files will now play fine again. This setup includes crossfading support as well, which is a nice bonus.