On current updated to the 15th of January I noticed that with pulse-audio the keyboard key mutes the sound correctly, but fails to unmute the sound. This despite looking like it does from the GUI. This is under xfce4 as I'm not using KDE. The same problem occurs if I just toggle mute/unmute using the GUI.

This appears to be a pulse-audio specific issue. The solution posted in this Ubuntu thread unmutes the sound correctly.

For clarity: running either of the following command unmutes the sound:
It might be that the solution is provided in that Ubuntu-thread, but that isn't clear to me as xfce4-volumed is gone now.

btw) the latest changes to current haven't hit my mirror, so I couldn't try with those to see if that resolves this problem.

Curiously, I was having this problem on my Thinkpad up until kernel 4.1.15 at which point it was sorted. And seems to still work correctly with PA.

The cause was a behavior where muting would mute *both* the Master channel and the Speaker channel, but un-muting would only unmute the Master channel.

Not sure any of those observations are relevant to you, but just in case....

Dave