After connecting, and disconnecting, an HDMI display, browsers lose sound forever (both Firefox and Chrome)

In a situation like this, VLC displays 2 audio devices and allows to select one, but browsers are stuck with the wrong one.

Quoting from the official documentation:

How to set the default device

If you, for example, plug in a new sound card and want it to be the default device from now on, you can tell PulseAudio to change the fallback sink to the new sound card, but that will only have effect on programs that PulseAudio hasn't seen before. For this particular use case the current (as of 2010-01-23) state-of-the-art method to move everything to the new sound card is to use gnome-volume-control version 2.28, which modifies the stream-restore database when you set some device as the default.

The second alternative is to use pavucontrol to move streams to the new sound card whenever you notice that some program is playing to a wrong sound card.

The third alternative is to first set the fallback device, then stop PulseAudio, then erase the stream-restore database by deleting the file containing the string "stream-volumes" in its name in the ~/.pulse directory, and finally restarting PulseAudio.

There is also the fourth option to make default sink and source to work to all applications by disabling stream target device reading from the stream cache. Simply modify the load-module module-stream-restore line that should already be in your /etc/pulse/, so that it looks like this:

load-module module-stream-restore restore_device=false

Then restart PulseAudio.

Now all opened streams will open to default sink or source even in the case that there is a record in the stream-volumes for the stream. Now you can plug an USB audio device and set it as default sink and source and all opened streams (new and or already seen) will go to the new device. Only streams that are open need to be moved by pavucontrol or re-opened.

Note: gnome-volume-control is not used in Fedora 23, its function is taken over by Gnome Shell.