No sound in Flash videos

If you have no sound in Flash videos but in all other programs, there are two possibilities:

1. the PCM output is set to silence
   • check volume setting with alsamixer
2. you might have a second audio device (e.g. a Webcam with a microphone) which gets preferred by ALSA
   • fix it by unplugging it, or
   • force ALSA to use another device, see ALSA FAQ#26
   • set Skype → Options → Sound Devices to [ ] Allow Skype to automatically adjust my mixer levels
3. some sound via PulseAudio is currently playing (see ALSA → PulseAudio for Flash and others)

Bug report is #396558.