If you have no sound in Flash videos but in all other programs, there are two possibilities:
- the PCM output is set to silence
- check volume setting with
alsamixer
- check volume setting with
- 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
- some sound via PulseAudio is currently playing (see ALSA → PulseAudio for Flash and others)
Bug report is #396558.