My volume keys are not working in xfce. When I used them it shows an indication on the screen that the volume is being changed, but the volume is not changing at all (same thing with the mute key).

The weird thing is that when I go from xfce to ubuntu the keys are now working fine.

I don't know if this matters but I have a sound card in my computer.

How can I make the volume keys work in xfce?