After mucking around with Amarok for about half an hour, I remembered that by default, Ubuntu doesn’t include support for “non-free” codecs like mp3 or dvd playback. You have to install the ubuntu-restricted-extras package to enable these abilities. Full terminal command is:
sudo apt-get install ubuntu-restricted-extras
It’s easy enough to do, but annoying that this isn’t included by default. Still, it’s a small price to pay to use free (as in speech) software. Especially when it’s also free (as in beer).