National Instruments has published a media player example
here.
You can just change the call to MediaPlayer_IMediaPlayer2SetFileName() to pass the path of your MP3 file.
The example was created in CVI 7.0. I loaded it into CVI 6.0 and it gave me a warning that the UIR was created in a later version, but it ran fine and played a couple of MP3's that I tried. I'm running CVI 6, Media Player 7, Windows 2000.