I noticed that on several Dell laptops that I’ve upgraded to Debian 8 ‘Jessie’ with the XFCE desktop environment, that the keyboard mute button had stopped working. Or rather, the button mutes the audio just fine, but pressing it again doesn’t actually unmute again. To get audio back, you have to manually invoke alsamixer and unmute from there.

After more searching than it seemed this problem ought to require, I found this StackExchange answer, which references a post on Rony Lutsky’s blog which gave me the solution.

It turns out that the fix is remarkably simple:

sudo apt-get install gstreamer0.10-pulseaudio

You can then, if you want, verify that it worked by running xfconf-query -lc xfce4-mixer before and after installing gstreamer0.10, but this isn’t a key part of the process.

From what I can tell, the issue is a missing dependency in one of the XFCE audio packages, but I’m damned if I know which one exactly.