A solution may be to make libxine1-misc-plugins a dependency on the Amarok package. Another solution may be to edit the install-mp3 script that is installed to /usr/lib/amarok/install-mp3/ and edit that so that this plugin automatically get's installed too.
Addendum:
A solution may be to make libxine1- misc-plugins a dependency on the Amarok package. Another solution may be to edit the install-mp3 script that is installed to /usr/lib/ amarok/ install- mp3/ and edit that so that this plugin automatically get's installed too.