1. startup parameters:
You can use the startup parameters that come with vlc. for example:
"C:\Program Files\VideoLAN\VLC\vlc.exe" --started-from-file "C:\blabla.mp3" --qt-volume-complete --volume="1000" --force-dolby-surround="1"
starts vlc with the specified mp3, extends the volume from 0-400, set the volume to 1000 (98%) and force dolby surround sound.
complete info on the startup parameters is at URL: http://wiki.videolan.org/VLC_command-line_help.

2. Integrate with the 4GL
You can also use the VLPlugin2 from the OCX button on the palette (AppBuilder). 
Drop it on your window. Doubleclick to set the file (BaseURL) to play, set the volume, etc. With this OCX functionality it is, afai know, only possible to play the file you assigned to the BaseURL property before starting the program; you cannot assign the file to play after instantiation.

So you're better of with the first option.

hope this helps.
regards,
Will