Packages for the release of Amarok 2.6 are available for Kubuntu 12.04. You can get it from the Kubuntu Backports PPA (alongside KDE SC 4.9). They are also in our development release.
Bugs in the packaging should be reported to kubuntu-ppa on Launchpad. Bugs in the software to KDE.
To update, use the Software Repository Guide to add the following repository to your software sources list:
ppa:kubuntu-ppa/backports
and install amarok and phonon-backend-gstreamer.