It's the button to adjust the size of the audio buffer, which should only appear if the Advanced Options are on. The button shouldn't be seen unless they are, but when they are it does have a purpose.
Actually, its a button i added right at the end of development to test our music system. When the music system is enabled, text is shown next to this button which is the name of the currently playing track. The button moves to the next track, so that we could more easily determine which tracks we wanted to include.
I see. Curiously enough, I can see it with the advanced options deselected. It doesn't matter either way, at least I understand its function now. Appreciate the clarification, Gregor.
Edit: My original reply was to Gregor's post prior to Alex stealthily sneaking in a reply. :)