Volume control: sound is very loud

[Display]

On a brand new LMDE 64-bit XFCE install. I have problems with volume control:
When I use Firefox, the sound volume is okay with Flash. When I modify the volume level from the Flash player, everything is okay.
But when I use VLC or Totem (I didn't tested another one) the volume control of the application is linked to the volume control of the widget in the taskbar: if I set the volume at 50% inside VLC, then the taskbar widget is now set to 50% too. This is very anoying because this modify the volume for every other applications too.

I don't have this problem on the same computer with Mint 13 + Mate.

--> how can I "disconnect" the application's volume level from the taskbar widget volume level ?