I found an issue with using the mute button on a dell laptop with media controls, it mutes the master volume, the speaker volume and the headphone volume in audio mixer. Using the same media control button only unmutes the master vol and not the rest of them.

Recently on my Sid install when changing between headphones and digital out I have found that the output being switched to is muted in PulseAudio. Strange that it would be happening in Jessie though.
My experience of sound on Linux has been that if there's a problem it's usually PulseAudio.

I maybe had a similar problem this solution worked and might help someone here. Debian Jessie 8, 64 bit, XFCE 4.10 sound worked fine since install for a few months. Suddenly after a reboot it ceased working. I booted up using my trusty trouble-shooting backup OS thumb drive and sound worked, so it wasn't a bad sound card. Next booted back into non-sound functioning OS and installed pavucontrol (PulseAudio Volume Control). Then play something in an audio or video player; SMPlayer, Clementine, youtube, whatever. Start PulseAudio Volume Control either from Whisker menu or run in a terminal: pavucontrol. On the first tab 'Playback' it had defaulted to "GT216 HDMI Audio Controller Digital Stereo (HDMI)". Change it to "Built-in Audio Analog Stereo" makes the sound work again. How it was messed up in the first place, I don't know. Only two things I had done differently prior to the sound trouble was to install 'devede' and uninstalled it, and I also tried the Audio>Filters>Extrastereo setting in SMPlayer. After using pavucontrol it now survives a reboot and works fine.