try this procedure

1) Download the latest Realplayer from the HelixCommunity.org or from Real.com. We had tested with RealPlayer11Gold.bin. Remember it is the Realplayer and not the Helix Player. The file that we had downloaded and tested was
http://www.real.com/realcom/R?href=http://forms.real.com/real/player/download.html?f=unix/RealPlayer11GOLD.bin. When prompted save the file to your home folder

2) Set the executable bit of the file and then run the installer.

chmod +x RealPlayer11GOLD.bin

sudo ./RealPlayer11GOLD.bin

Proceed with the installation and leave the default options as they are. If all goes well your Real Player will be installed to /opt/real/RealPlayer.

3) However there has been some directory level changes with Firefox 3 which comes with Ubuntu 8.04 and hence the Real Player plugin will not work in Firefox. You can easily fix this problem by running the following commands

cd /usr/lib/firefox-addons/plugins

sudo ln -s /opt/real/RealPlayer/mozilla/nphelix.xpt nphelix.xpt

sudo ln -s /opt/real/RealPlayer/mozilla/nphelix.so nphelix.so

sudo mv /usr/lib/totem/gstreamer/libtotem-complex-plugin.so ~/.

Now check if the Real Player plugin is installed properly by typing in the following in the Firefox address bar - about:plugins. If you had followed the instructions correctly and you have Ubuntu 8.04 and have used Real Player 11 Gold you should be able to see the following in the about:plugins page

Helix DNA Plugin: RealPlayer G2 Plug-In Compatible

File name: /usr/local/RealPlayer/mozilla/nphelix.so

Helix DNA Plugin: RealPlayer G2 Plug-In Compatible version 0.4.0.626 built with gcc 3.2.0 on Jul 26 2007

4) Now go to raaga.com and see if the player is working.

I have been trying to play songs from the popular website in India http://raaga.com which is real audio format. Even, I have tried playing with realplayer mozilla plugins nphelix.so & nphelix.xpt , it was no use.
It will be really useful if you help out.

Thanks,
Abhinay