Normally my laptop is on my desk at home plugged into a sound system, so I never have to change the volume. However I’m currently on holiday, so that means I’m carrying my laptop around. Last night, I had the audacity to lower the volume on my machine. After all, nobody wants to wake up their family at 2am with “The history of the USSR set to Tetris.flv”. Using the media keys on my laptop did nothing. Lowering the sound in KMix did nothing. Muting in KMix did nothing. I figured that something had gone wrong with KMix and maybe I should re-open it. Well, it turns out that was a big goddamn mistake, because that resulted in me having no sound.
It took about 30 minutes to figure out, but the solution ended up being unmuting my headphone channel in alsamixer. It looks like for whatever reason, alsamixer and KMix were set to different master channels (headphone/speaker and HDMI, respectively), thus giving KMix (and my media keys) no actual control over volume.