Just a note, the alpha isn't suitable for the average desktop machine - it's a (very early) preview of what F11 will look like. If you're looking for something a bit more stable, I recommend you install Fedora 10 for now.
Anyways, first step is installing RPM Fusion for rawhide/testing releases:
rpm -Uhv http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-rawhide.noarch.rpm http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-rawhide.noarch.rpm
Next, you'll need to install a few gstreamer plugin packages which include the codecs you need:
yum install gstreamer-plugins-bad gstreamer-plugins-ugly