If the width of the mini player is too small the volume slider (or whatever that slider should be since it doesn't do anything) is displayed ontop or behind the other buttons.
Just a suggestion. This audio library is used by many programs (example: MusicBee) and has already many functions you may want to integrate in the music manager. (CD Ripper, Internet Streaming, WASAPI Output, Effects,...) It's not free for shareware use but reactively cheap.