Volume buttons not working when the screen is locked

Hello!

I would like to use the volume buttons even if the screen is locked. This is especially relevant for times when I'm listening to music and the volume changes from one song to the next or if I want to do a quick mute when the phone rings and then unmute after the conversation.

Currently I have to reenter my password and only then I can interact with the volume.

1) The version and edition of Linux Mint you are using (if you are not sure, open a terminal and run "mintwelcome").
14(nadia) Cinnamon 64-bit

2) What you did for the problem to happen, and how to reproduce it.
Listen to music, wait for the screen to dimm and lock and then press the mute button and the volume buttons.

3) What happened.
The volume stayed the same.

4) What you expected to happen instead.
I expected the volume to mute or change.

5) If the problem happened once, sometimes, or always.
Always.

Thanks,
Alex