Hey, it seems that, starting 4.8, xfce4-mixer doesn't have a minimize button anymore. I can't think of a good reason for that so I guess it's a bug.
This is caused by xfwm4 policy change regarding non-transient dialog windows (the xfce4-mixer is subclassed from XfceTitled dialog which is by default a dialog type window), see bug #9116. Arguably the xfce4-mixer main window should rather be of normal type so that might be the appropriate fix for this bug.
All the code has to do is be to call: gtk_window_set_type_hint (GDK_WINDOW_TYPE_HINT_NORMAL); than it will act again like a casual window, and not like a dialog, which is usually a part of an application.
*The code needs to call: (arrr, my creepy english)
Fixed in commit 3e0def60db29746e501586d36d33ea305540a2e8.
*** Bug 8715 has been marked as a duplicate of this bug. ***