I get sound just fine using Chrome (e.g. YouTube) or with Rhythmbox playing an MP3.
However, VLC, Xine, and the default Ubuntu "Videos" make no sound, whether with MP4, FLV, or MP3. How do I fix this?
Ubuntu 14.10 Lenono M30-70
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Same thing happened to me with VLC and Totem in Ubuntu 16.04. However in some other players like mpv sound was working fine.
I had the feeling it had something to do with last time I opened VLC. I was watching a movie on the TV (by HDMI connection) so back then I configured the audio to be played by HDMI output. I powered off the laptop without reverting the configuration.
But now I was not connected to HDMI and in sound configuration only internal speakers were showing.
In the end, the configuration option to blame was to be found in VLC itself. While playing, Audio > Audio device > (select your internal speakers). I seems it did not revert itself to internal speakers, because HDMI output was still selected. Now even Totem is working.
I hope this experience helps someone!
According to help.ubuntu::
No sound If you don't hear any sound, try: (a) click System -> Preferences -> Sounds. Then click a sound in the bottom list, and make sure that it's not already selected. You should hear the sound you selected. If not, make sure your speakers are working and that your volume is up. Try clicking another sound in the list, after ajusting your volume. Also make sure your speakers are working. Once you're done, select the sound it was on before and click OK. (b) downloading VLC Media Player by clicking Applications -> Ubuntu Software Centre or Applications -> Add/Remove Appications. Search for "VLC Media Player" and download the program with the orange cone icon. If you don't see an orange cone, the icon might be a cardboard box with all kinds of stuff sticking out of it. In that case, look at the discriptions. Install VLC and then use it to play your movie or DVD.
If this doesn't work, just try to install media codes.
If you changed manually once the output in your sound settings (for me that was an HDMI audio controller to have the sound of my home cinema), disconnecting the HDMI cable and trying to hear back your built in audio card does not work. I had to connect back to the HDMI cable, change manually the output to my built in audio and then it worked.
I'm running Debian jessie and had the exact same problem as original poster, Audacious & web browsers played audio, but VLC and GNOME Mplayer were muted. I had previously made a change to VLC preferences to capture stills from video. As far as I know, the only changes were the path and frame rate for the scene filter, plus enabled the scene video filter. When I tried to watch a movie with GNOME Mplayer, audio was muted. Audacious played the audio of the exact same mp4 no problem. I disabled the scene video filter and checked all the audio settings (default) in VLC. Still VLC and GNOME Mplayer did not have audio.
Since I usually run just the defaults in VLC, I clicked reset preferences, and I now have audio.