I'll start by saying I've solved the problem, but it had me scratching my head for a while.

I'd been messing about with my multimedia settings in Plasma 5 (Kubuntu 16.04), when all playback started happening about ten times too fast. Firefox playing an HTML5 video on YouTube, Google Chrome using its built-in Flash to play news clips on bbc.co.uk/news, mpv playing a local video or audio file -- all played too fast, and there was no sound. I could use the keyboard controls to slow down the mpv playback to about normal speed, but still there was no sound.

I'm using a desktop computer with the sound system built in to the motherboard. Eventually I realised that in the Multimedia System Setting, Audio and Video button, Audio Hardware Setup tab, I'd accidentally changed the profile from Analogue Stero Output to Analogue Surround 2.1 Output. Changing it back fixed the playback problem.

But what happened to speed up the video playback?! Is this a bug? It's not expected behaviour -- I'd expect in the worst case there to be no sound, and the video to be unaffected.

Incidentally, I was playing around in the first place because I wanted to adjust the left-right balance. KMix seems to have disappeared from the distribution. Is there no way now to adjust the balance?

Hi. I think it happens when default sound device is set to HDMI audio instead of sound card.