The tracker is a pretty smart daemon that runs in the background, and indexes all your music, photos and so on. The problem for me, was that this process either took all the cpu all the time, or it did nothing, and no music or photos was being indexed…
I found that the problem is the “Extra decoder-support” package. Remove it, then run the following command in a terminal window: “tracker-processes -r”, this will remove your index. Then start the media-player, and it should start indexing your files again.