Hmm. Looks like the Kubuntu page hasn't been updated to show this latest release (maybe packages aren't available yet for Kubuntu). However, if you click Download, then Kubuntu, you will be able to install the 1.4.8 release of Amarok.

As it is, the general policy with the regular Ubuntu repositories, as I understand it, is in general not to include major release updates of packages. This was the case with firefox for ages, and if you wanted the latest version of firefox, you needed to download a package from somewhere else. One solution to this, but I think with slightly less guaranteed stability/security, is to include the backports repository in your apt/synaptic. Backports is where newer upgrades of packages are 'ported back' to the current stable Ubuntu version, in order to allow users to get the latest versions of their packages, even when these are not necessarily in the main ubuntu repos.

For more info about backports, see https://help.ubuntu.com/community/UbuntuBackports