Dtyjlui

None of the HDMI output are working, all saying "dummy output", it was working yesterday, but when I use the headphones it became soundless, with or without the headphones.
Thank you already!

I had a similar issue happen where suddenly (I have no idea what I updated, or what changed in the system) I could not get sound.

I did the whole pulseaudio -k && sudo alsa force-reload thing. I didn't have any cards listed from pacmd list-cards but I could see the device with commands like:
lspci -nnk | grep -A2 Audio

I even installed a newer kernel (4.8+ since it apparently has good support for my particular machine).

In the end, I rebooted into the bios and disabled the built-in audio, applied those changes, rebooted to Ubuntu, then rebooted again into the bios to re-enable the audio. Once back into Ubuntu after that, everything seemed to work great!

Don't know if this will help with other people looking for solutions to their problems. I was using a Dell XPS 13 (9350), but might be something for others to at least try.

Update: I forgot you're required to have 10 points to do that. Try this.

pacmd list-cards

From there locate your audio device, and set it as the default.

An example of this would be something like

pacmd set-card-profile 2 output:analog-stereo

You can find more information on this in the following link.

Change default sound device

Unfortunatley it's difficult for me to really help you out without either some form of terminal output or a picture, a list of steps you've taken etc..

So another thing you could do is (this will make sure you get to where I tell you to go)

Open your terminal and type

unity-control-settings

That command opens the system settings. From there go to hardware section (middle) and select Sound/Sound Settings.

I'm sure you did this already, this is what I was saying before. Just to make sure we cover the basics I'll add it in again any way.

Go through each tab Output, Input (mainly those two), Sound effects, and applications. Select and set the default audio device (just click on it). You can even test the sound to verify it's there.

The reason I bring this up again is because you say it happened after you used headphones which sounds like the default device was changed.

Make sure if you're using any form of external hardware the required drivers for them are in fact installed. If you still cannot get sound, Send me a Private message with a picture or some terminal out put.

working for dell +2. Mine is Dell inspiron 5570. bios->disable audio -> boot to ubuntu -> restart -> bios -> enable aduio -> boot to ubuntu

Technically speaking the sound disappeared in the new accounts that have been created on the same machine and the same Ubuntu Version.
The weird part is that no particular changes have been made to any of those accounts. By that I mean some files were copied from one folder to an other. No change in addition from the main (admin/ root) account either. The sound kept disappearing progressively from every account which is not root. I created a new account just to see what would happen. The sound disappears after a couple of hours.

To fix that some methods require a sudo privileges. Yet it doesn't work with the users which aren't "sudoers". To be more precise they aren't in the sodoers file.

And for some reason the computer "see's" the soundcard while the soundmixer doesn't seem to.

The weirdest was after.
The sound came back as unexpected as it left.
Yes, I unchecked the members (users) from the audio group (whom I started to put in after the sound disappeared) and
yes, I restarted the computer from the main account.
It seems that the issue might come from the way the user have been logged out the one's from each others.