I recently upgraded from 8.10 to 9.04, and one of the changes that came with this was an "upgrade" to Amarok 2. Although this is the latest version of Amarok, the developers made a similar change to the upgrade from KDE 3.5 to 4.0 where by removing a lot of features and rebuilding important components, they have created a strong base to move forward with.
One of the side effects of this change for me was that my keyboard and volume sliders no longer affected Amarok. The problem is relatively simple; after the upgrade, Amarok is only affected by the Master Channel, rather than PCM (which I believe used to be the default). The solution to the problem is to right click on the volume icon, and choose "Select Master Channel...". Then select the radio button at the top next to "Master" and hit okay.
Now when you use your standard volume keyboard shortcuts, or the volume slider, Amarok will be affected as expected.