No sound from internal speakers after using HDMI output

I fixed with this

$ rm -rf /tmp/pulse* ~/.pulse* ~/.config/pulse
$ pulseaudio -k
$ pulseaudio --start

I had the same problem, here is how to allow internal sound after using HDMI output - this worked for me.

In the hidden cache folder in your home directory, there will be a file called something like event-sound-cache.tdb.641c9db826a44f28bb9596d6df3020c2.x86_64-pc-linux-gnu. Just find the file and remove it. Now, your internal sound should work.

In a terminal, from home directory:

cd .cache
ls -a

Now look for the file starting with event-sound-cache.tdb. For me, it was event-sound-cache.tdb.641c9db826a44f28bb9596d6df3020c2.x86_64-pc-linux-gnu

Now, in the terminal, type:

rm <file name here>

For me, my input looked like this:

rm event-sound-cache.tdb.641c9db826a44f28bb9596d6df3020c2.x86_64-pc-linux-gnu

Note that the event-sound-cache.tdb file with be unique to your machine-id and OS.

Your internal sound should now work. If the sound still does not work, go to the sound application and re-select Speakers (Built-in Audio) as the option for sound.

I had to find another solution since clear_night_sky's solution didn't work for me.

I went to Settings -> Sound, and switched between the "Input" and "Output" tabs and voilá. Sound was back. Hope that can help someone too.