Xfce4-mixer package is missing? How can I control volume?

I was having too many issues with gdm3 on my new Librem 13", so I went to xfce4, a window manager I’m more familiar with. However, and perhaps unsurprisingly, certain things like volume control no longer work. xfce4 is usually installed with a plugin (I think the package is called xfce4-mixer) but this appears to be missing. Does anyone know how I can get the volume control to work again with xfce4?

Thanks much,