Not sure about the media-keys/hardware configBut
On the panel you have a mixer control (default in Mint's I believe). You can click on and bring up a 'mini-mixer'.
Make sure sound card selected (alsa-playback, pulseadio-playback ect). Selectet control to show (will save & show from now on). The master control usually does the trick.
The icon below (looks like speaker) toggles mute on and off (normally show red x to show state in also).