Ubuntu – Ubuntu’s stopped playing music/video files & Youtube

Two days ago (after an Ubuntu base update, iirc), all music and video files on my computer stopped playing. Clementine and Videos open them, but won't play them – no error message, no request for plugins.
At the same time, YouTube clips have stopped playing – I just get the thumbnail, the rotating "waiting" sign and the message "if playback doesn't begin shortly, try restarting your device" (which, of course, doesn't work).
This is Ubuntu 14.04.
Details:

• I do have gstreamer0.10-plugins-good and ubuntu-restricted-extras. Tried reinstalling (together with Clementine and Videos) – no good.
• Videos can clearly "read" the video file, since I can navigate through it and the still picture shown changes.
• My speakers are recognised.
• Pressing "pause" then "play" again doesn't work.
• Flash sound does work! Interestingly enough, right after the update, I could get no sound on clips streamed in a Flash window (they were running though, the video was fine). After restarting, the sound from streaming works just fine, but music/video files and YouTube clips don't.
• I didn't install anything new or make any changes to Ubuntu before this happened.