Before you ask:
To program the convenience key(s), go to Options > Screen/Keyboard. Within this menu you will be able to select Left/Right Side Convenience Key Opens and then choose the Native OS or 3rd party application you want launched when the convenience key is clicked. USE the convenience keys - they are HANDY!
1. If you have a song playing and are in another screen or app, you can also press the Mute key on the top edge of the device which will pause it. Press it again and it will resume. It doesn't just mute/unmute it.
2. Additionally, you can press the Applications/Menu key to the left of the track ball which show Now Playing the the menu. Click on that and it will take you to the Media application.
3. You can also press & hold the Alt key then press the Escape key (to the right of the track ball) to scroll through the open applications. The Media application and Now Playing will also show up there.