The toolbar is divided into 2 parts:

   The left part contains general, non-customizable buttons:

The first button (from left to right) will display a list of the clipboards when the mouse button is held down.  The next four buttons grouped together stop, pause, play, and fast-forward a song respectively.  The next two buttons are the master volume control and the song speed control.  When the mouse button is held down over these buttons, a slider control will pop-up beneath the button.

  The right part of the toolbar contains customizable function buttons.  To arrange these buttons, choose “Edit Toolbar” from the Edit menu.