I just spent an hour fannying around with Amarok because it suddenly refused to play Ogg Vorbis encoded files. The correct solution turned out to be in this thread on the Ubuntu forums. The problem I was getting in Amarok was a message like this every time I tried to add an .ogg file to the playlist:

Some media could not be loaded (not playable)

Several of the solutions I came across recommended removing my Amarok profile, which then meant a good ten minutes waiting for it to re-scan my collection before I could see if the solution worked. Which it didn't.

A few more helpful solutions pointed the finger at Xine, the sound engine which Amarok uses, though most of the solutions were based around installing missing codec packages I already had (I was playing these same files in Amarok with no problems last week). However, when I tried playing the files in Xine directly I got another error message about there being 'no demuxer plugin available'. Again I spent some time trawling through solutions which blamed Grip for using incompatible versions of the Ogg Vorbis codec, or generating incompatible ID3 tags, but these were unhelpful.

The solution, when I found it in the forum thread above (which links to this Fedora related blog post), was very simple - exit Xine/Amarok, remove the catalog.cache and restart:

rm .xine/catalog.cache